From: Muchun Song <hidden> Date: 2021-01-06 14:21:15
Hi all,
This patch series will free some vmemmap pages(struct page structures)
associated with each hugetlbpage when preallocated to save memory.
In order to reduce the difficulty of the first version of code review.
From this version, we disable PMD/huge page mapping of vmemmap if this
feature was enabled. This accutualy eliminate a bunch of the complex code
doing page table manipulation. When this patch series is solid, we cam add
the code of vmemmap page table manipulation in the future.
The struct page structures (page structs) are used to describe a physical
page frame. By default, there is a one-to-one mapping from a page frame to
it's corresponding page struct.
The HugeTLB pages consist of multiple base page size pages and is supported
by many architectures. See hugetlbpage.rst in the Documentation directory
for more details. On the x86 architecture, HugeTLB pages of size 2MB and 1GB
are currently supported. Since the base page size on x86 is 4KB, a 2MB
HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of
4096 base pages. For each base page, there is a corresponding page struct.
Within the HugeTLB subsystem, only the first 4 page structs are used to
contain unique information about a HugeTLB page. HUGETLB_CGROUP_MIN_ORDER
provides this upper limit. The only 'useful' information in the remaining
page structs is the compound_head field, and this field is the same for all
tail pages.
By removing redundant page structs for HugeTLB pages, memory can returned to
the buddy allocator for other uses.
When the system boot up, every 2M HugeTLB has 512 struct page structs which
size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE).
HugeTLB struct pages(8 pages) page frame(8 pages)
+-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
| | | 0 | -------------> | 0 |
| | +-----------+ +-----------+
| | | 1 | -------------> | 1 |
| | +-----------+ +-----------+
| | | 2 | -------------> | 2 |
| | +-----------+ +-----------+
| | | 3 | -------------> | 3 |
| | +-----------+ +-----------+
| | | 4 | -------------> | 4 |
| 2MB | +-----------+ +-----------+
| | | 5 | -------------> | 5 |
| | +-----------+ +-----------+
| | | 6 | -------------> | 6 |
| | +-----------+ +-----------+
| | | 7 | -------------> | 7 |
| | +-----------+ +-----------+
| |
| |
| |
+-----------+
The value of page->compound_head is the same for all tail pages. The first
page of page structs (page 0) associated with the HugeTLB page contains the 4
page structs necessary to describe the HugeTLB. The only use of the remaining
pages of page structs (page 1 to page 7) is to point to page->compound_head.
Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
will be used for each HugeTLB page. This will allow us to free the remaining
6 pages to the buddy allocator.
Here is how things look after remapping.
HugeTLB struct pages(8 pages) page frame(8 pages)
+-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
| | | 0 | -------------> | 0 |
| | +-----------+ +-----------+
| | | 1 | -------------> | 1 |
| | +-----------+ +-----------+
| | | 2 | ----------------^ ^ ^ ^ ^ ^
| | +-----------+ | | | | |
| | | 3 | ------------------+ | | | |
| | +-----------+ | | | |
| | | 4 | --------------------+ | | |
| 2MB | +-----------+ | | |
| | | 5 | ----------------------+ | |
| | +-----------+ | |
| | | 6 | ------------------------+ |
| | +-----------+ |
| | | 7 | --------------------------+
| | +-----------+
| |
| |
| |
+-----------+
When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
vmemmap pages and restore the previous mapping relationship.
Apart from 2MB HugeTLB page, we also have 1GB HugeTLB page. It is similar
to the 2MB HugeTLB page. We also can use this approach to free the vmemmap
pages.
In this case, for the 1GB HugeTLB page, we can save 4094 pages. This is a
very substantial gain. On our server, run some SPDK/QEMU applications which
will use 1024GB hugetlbpage. With this feature enabled, we can save ~16GB
(1G hugepage)/~12GB (2MB hugepage) memory.
Because there are vmemmap page tables reconstruction on the freeing/allocating
path, it increases some overhead. Here are some overhead analysis.
1) Allocating 10240 2MB hugetlb pages.
a) With this patch series applied:
# time echo 10240 > /proc/sys/vm/nr_hugepages
real 0m0.166s
user 0m0.000s
sys 0m0.166s
# bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; } kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
Attaching 2 probes...
@latency:
[8K, 16K) 8360 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[16K, 32K) 1868 |@@@@@@@@@@@ |
[32K, 64K) 10 | |
[64K, 128K) 2 | |
b) Without this patch series:
# time echo 10240 > /proc/sys/vm/nr_hugepages
real 0m0.066s
user 0m0.000s
sys 0m0.066s
# bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; } kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
Attaching 2 probes...
@latency:
[4K, 8K) 10176 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[8K, 16K) 62 | |
[16K, 32K) 2 | |
Summarize: this feature is about ~2x slower than before.
2) Freeing 10240 2MB hugetlb pages.
a) With this patch series applied:
# time echo 0 > /proc/sys/vm/nr_hugepages
real 0m0.004s
user 0m0.000s
sys 0m0.002s
# bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; } kretprobe:__free_hugepage /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
Attaching 2 probes...
@latency:
[16K, 32K) 10240 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
b) Without this patch series:
# time echo 0 > /proc/sys/vm/nr_hugepages
real 0m0.077s
user 0m0.001s
sys 0m0.075s
# bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; } kretprobe:__free_hugepage /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
Attaching 2 probes...
@latency:
[4K, 8K) 9950 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[8K, 16K) 287 |@ |
[16K, 32K) 3 | |
Summarize: The overhead of __free_hugepage is about ~2-4x slower than before.
But according to the allocation test above, I think that here is
also ~2x slower than before.
But why the 'real' time of patched is smaller than before? Because
In this patch series, the freeing hugetlb is asynchronous(through
kwoker).
Although the overhead has increased, the overhead is not significant. Like Mike
said, "However, remember that the majority of use cases create hugetlb pages at
or shortly after boot time and add them to the pool. So, additional overhead is
at pool creation time. There is no change to 'normal run time' operations of
getting a page from or returning a page to the pool (think page fault/unmap)".
Todo:
- Free all of the tail vmemmap pages
Now for the 2MB HugrTLB page, we only free 6 vmemmap pages. we really can
free 7 vmemmap pages. In this case, we can see 8 of the 512 struct page
structures has beed set PG_head flag. If we can adjust compound_head()
slightly and make compound_head() return the real head struct page when
the parameter is the tail struct page but with PG_head flag set.
In order to make the code evolution route clearer. This feature can can be
a separate patch after this patchset is solid.
- Support for other architectures (e.g. aarch64).
- Enable PMD/huge page mapping of vmemmap even if this feature was enabled.
Changelog in v11 -> v12:
- Move VM_WARN_ON_PAGE to a separate patch.
- Call __free_hugepage() with hugetlb_lock (See patch #5.) to serialize
with dissolve_free_huge_page(). It is to prepare for patch #9.
- Introduce PageHugeInflight. See patch #9.
Changelog in v10 -> v11:
- Fix compiler error when !CONFIG_HUGETLB_PAGE_FREE_VMEMMAP.
- Rework some comments and commit changes.
- Rework vmemmap_remap_free() to 3 parameters.
Thanks to Oscar and Mike's suggestions and review.
Changelog in v9 -> v10:
- Fix a bug in patch #11. Thanks to Oscar for pointing that out.
- Rework some commit log or comments. Thanks Mike and Oscar for the suggestions.
- Drop VMEMMAP_TAIL_PAGE_REUSE in the patch #3.
Thank you very much Mike and Oscar for reviewing the code.
Changelog in v8 -> v9:
- Rework some code. Very thanks to Oscar.
- Put all the non-hugetlb vmemmap functions under sparsemem-vmemmap.c.
Changelog in v7 -> v8:
- Adjust the order of patches.
Very thanks to David and Oscar. Your suggestions are very valuable.
Changelog in v6 -> v7:
- Rebase to linux-next 20201130
- Do not use basepage mapping for vmemmap when this feature is disabled.
- Rework some patchs.
[PATCH v6 08/16] mm/hugetlb: Free the vmemmap pages associated with each hugetlb page
[PATCH v6 10/16] mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb page
Thanks to Oscar and Barry.
Changelog in v5 -> v6:
- Disable PMD/huge page mapping of vmemmap if this feature was enabled.
- Simplify the first version code.
Changelog in v4 -> v5:
- Rework somme comments and code in the [PATCH v4 04/21] and [PATCH v4 05/21].
Thanks to Mike and Oscar's suggestions.
Changelog in v3 -> v4:
- Move all the vmemmap functions to hugetlb_vmemmap.c.
- Make the CONFIG_HUGETLB_PAGE_FREE_VMEMMAP default to y, if we want to
disable this feature, we should disable it by a boot/kernel command line.
- Remove vmemmap_pgtable_{init, deposit, withdraw}() helper functions.
- Initialize page table lock for vmemmap through core_initcall mechanism.
Thanks for Mike and Oscar's suggestions.
Changelog in v2 -> v3:
- Rename some helps function name. Thanks Mike.
- Rework some code. Thanks Mike and Oscar.
- Remap the tail vmemmap page with PAGE_KERNEL_RO instead of PAGE_KERNEL.
Thanks Matthew.
- Add some overhead analysis in the cover letter.
- Use vmemap pmd table lock instead of a hugetlb specific global lock.
Changelog in v1 -> v2:
- Fix do not call dissolve_compound_page in alloc_huge_page_vmemmap().
- Fix some typo and code style problems.
- Remove unused handle_vmemmap_fault().
- Merge some commits to one commit suggested by Mike.
Muchun Song (13):
mm/memory_hotplug: Factor out bootmem core functions to bootmem_info.c
mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP
mm: Introduce VM_WARN_ON_PAGE macro
mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page
mm/hugetlb: Defer freeing of HugeTLB pages
mm/hugetlb: Allocate the vmemmap pages associated with each HugeTLB
page
mm/hugetlb: Set the PageHWPoison to the raw error page
mm/hugetlb: Flush work when dissolving a HugeTLB page
mm/hugetlb: Introduce PageHugeInflight
mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap
mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate
mm/hugetlb: Gather discrete indexes of tail page
mm/hugetlb: Optimize the code with the help of the compiler
Documentation/admin-guide/kernel-parameters.txt | 14 ++
Documentation/admin-guide/mm/hugetlbpage.rst | 3 +
arch/x86/mm/init_64.c | 13 +-
fs/Kconfig | 18 ++
include/linux/bootmem_info.h | 65 ++++++
include/linux/hugetlb.h | 37 ++++
include/linux/hugetlb_cgroup.h | 15 +-
include/linux/memory_hotplug.h | 27 ---
include/linux/mm.h | 5 +
include/linux/mmdebug.h | 8 +
mm/Makefile | 2 +
mm/bootmem_info.c | 124 +++++++++++
mm/hugetlb.c | 218 +++++++++++++++++--
mm/hugetlb_vmemmap.c | 278 ++++++++++++++++++++++++
mm/hugetlb_vmemmap.h | 45 ++++
mm/memory_hotplug.c | 116 ----------
mm/sparse-vmemmap.c | 256 ++++++++++++++++++++++
mm/sparse.c | 1 +
18 files changed, 1073 insertions(+), 172 deletions(-)
create mode 100644 include/linux/bootmem_info.h
create mode 100644 mm/bootmem_info.c
create mode 100644 mm/hugetlb_vmemmap.c
create mode 100644 mm/hugetlb_vmemmap.h
--
2.11.0
From: Muchun Song <hidden> Date: 2021-01-06 14:21:16
Move bootmem info registration common API to individual bootmem_info.c.
And we will use {get,put}_page_bootmem() to initialize the page for the
vmemmap pages or free the vmemmap pages to buddy in the later patch.
So move them out of CONFIG_MEMORY_HOTPLUG_SPARSE. This is just code
movement without any functional change.
Signed-off-by: Muchun Song <redacted>
Acked-by: Mike Kravetz <redacted>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: David Hildenbrand <redacted>
---
arch/x86/mm/init_64.c | 3 +-
include/linux/bootmem_info.h | 40 +++++++++++++
include/linux/memory_hotplug.h | 27 ---------
mm/Makefile | 1 +
mm/bootmem_info.c | 124 +++++++++++++++++++++++++++++++++++++++++
mm/memory_hotplug.c | 116 --------------------------------------
mm/sparse.c | 1 +
7 files changed, 168 insertions(+), 144 deletions(-)
create mode 100644 include/linux/bootmem_info.h
create mode 100644 mm/bootmem_info.c
@@ -1571,7 +1572,7 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,returnerr;}-#if defined(CONFIG_MEMORY_HOTPLUG_SPARSE) && defined(CONFIG_HAVE_BOOTMEM_INFO_NODE)+#ifdef CONFIG_HAVE_BOOTMEM_INFO_NODEvoidregister_page_bootmem_memmap(unsignedlongsection_nr,structpage*start_page,unsignedlongnr_pages){
@@ -33,18 +33,6 @@ struct vmem_altmap;___page;\})-/*-*Typesforfreebootmemstoredinpage->lru.next.Thesehavetobein-*somerandomrangeinunsignedlongspacefordebuggingpurposes.-*/-enum{-MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE=12,-SECTION_INFO=MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE,-MIX_SECTION_INFO,-NODE_INFO,-MEMORY_HOTPLUG_MAX_BOOTMEM_TYPE=NODE_INFO,-};-/* Types for control the zone type of onlined and offlined memory */enum{/* Offline the memory. */
From: Muchun Song <hidden> Date: 2021-01-06 14:21:40
Very similar to VM_WARN_ON_ONCE_PAGE and VM_BUG_ON_PAGE, add
VM_WARN_ON_PAGE macro.
Signed-off-by: Muchun Song <redacted>
---
include/linux/mmdebug.h | 8 ++++++++
1 file changed, 8 insertions(+)
From: Muchun Song <hidden> Date: 2021-01-06 14:21:40
The HUGETLB_PAGE_FREE_VMEMMAP option is used to enable the freeing
of unnecessary vmemmap associated with HugeTLB pages. The config
option is introduced early so that supporting code can be written
to depend on the option. The initial version of the code only
provides support for x86-64.
Like other code which frees vmemmap, this config option depends on
HAVE_BOOTMEM_INFO_NODE. The routine register_page_bootmem_info() is
used to register bootmem info. Therefore, make sure
register_page_bootmem_info is enabled if HUGETLB_PAGE_FREE_VMEMMAP
is defined.
Signed-off-by: Muchun Song <redacted>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Acked-by: Mike Kravetz <redacted>
---
arch/x86/mm/init_64.c | 2 +-
fs/Kconfig | 18 ++++++++++++++++++
2 files changed, 19 insertions(+), 1 deletion(-)
From: Muchun Song <hidden> Date: 2021-01-06 14:21:41
Every HugeTLB has more than one struct page structure. We __know__ that
we only use the first 4(HUGETLB_CGROUP_MIN_ORDER) struct page structures
to store metadata associated with each HugeTLB.
There are a lot of struct page structures associated with each HugeTLB
page. For tail pages, the value of compound_head is the same. So we can
reuse first page of tail page structures. We map the virtual addresses
of the remaining pages of tail page structures to the first tail page
struct, and then free these page frames. Therefore, we need to reserve
two pages as vmemmap areas.
When we allocate a HugeTLB page from the buddy, we can free some vmemmap
pages associated with each HugeTLB page. It is more appropriate to do it
in the prep_new_huge_page().
The free_vmemmap_pages_per_hpage(), which indicates how many vmemmap
pages associated with a HugeTLB page can be freed, returns zero for
now, which means the feature is disabled. We will enable it once all
the infrastructure is there.
Signed-off-by: Muchun Song <redacted>
---
include/linux/bootmem_info.h | 27 +++++-
include/linux/mm.h | 3 +
mm/Makefile | 1 +
mm/hugetlb.c | 3 +
mm/hugetlb_vmemmap.c | 211 +++++++++++++++++++++++++++++++++++++++++++
mm/hugetlb_vmemmap.h | 20 ++++
mm/sparse-vmemmap.c | 182 +++++++++++++++++++++++++++++++++++++
7 files changed, 446 insertions(+), 1 deletion(-)
create mode 100644 mm/hugetlb_vmemmap.c
create mode 100644 mm/hugetlb_vmemmap.h
@@ -27,8 +27,190 @@#include<linux/spinlock.h>#include<linux/vmalloc.h>#include<linux/sched.h>+#include<linux/pgtable.h>+#include<linux/bootmem_info.h>+#include<asm/dma.h>#include<asm/pgalloc.h>+#include<asm/tlbflush.h>++/**+*vmemmap_remap_walk-walkvmemmappagetable+*+*@remap_pte:calledforeachnon-emptyPTE(lowest-level)entry.+*@reuse_page:thepagewhichisreusedforthetailvmemmappages.+*@reuse_addr:thevirtualaddressofthe@reuse_pagepage.+*@vmemmap_pages:thelistheadofthevmemmappagesthatcanbefreed.+*/+structvmemmap_remap_walk{+void(*remap_pte)(pte_t*pte,unsignedlongaddr,+structvmemmap_remap_walk*walk);+structpage*reuse_page;+unsignedlongreuse_addr;+structlist_head*vmemmap_pages;+};++staticvoidvmemmap_pte_range(pmd_t*pmd,unsignedlongaddr,+unsignedlongend,+structvmemmap_remap_walk*walk)+{+pte_t*pte;++pte=pte_offset_kernel(pmd,addr);++/*+*Theroutineofvmemmappagetablewalkinghasthefollowingrules:+*+*-reuseaddressispartoftherangethatwearewalking.+*-reuse_pageisfound'first'intablewalkbeforewestart+*remapping(whichiscalling@walk->remap_pte).+*/+if(walk->reuse_addr==addr){+BUG_ON(pte_none(*pte));++walk->reuse_page=pte_page(*pte++);+addr+=PAGE_SIZE;+}++for(;addr!=end;addr+=PAGE_SIZE,pte++){+BUG_ON(pte_none(*pte));++walk->remap_pte(pte,addr,walk);+}+}++staticvoidvmemmap_pmd_range(pud_t*pud,unsignedlongaddr,+unsignedlongend,+structvmemmap_remap_walk*walk)+{+pmd_t*pmd;+unsignedlongnext;++pmd=pmd_offset(pud,addr);+do{+BUG_ON(pmd_none(*pmd));++next=pmd_addr_end(addr,end);+vmemmap_pte_range(pmd,addr,next,walk);+}while(pmd++,addr=next,addr!=end);+}++staticvoidvmemmap_pud_range(p4d_t*p4d,unsignedlongaddr,+unsignedlongend,+structvmemmap_remap_walk*walk)+{+pud_t*pud;+unsignedlongnext;++pud=pud_offset(p4d,addr);+do{+BUG_ON(pud_none(*pud));++next=pud_addr_end(addr,end);+vmemmap_pmd_range(pud,addr,next,walk);+}while(pud++,addr=next,addr!=end);+}++staticvoidvmemmap_p4d_range(pgd_t*pgd,unsignedlongaddr,+unsignedlongend,+structvmemmap_remap_walk*walk)+{+p4d_t*p4d;+unsignedlongnext;++p4d=p4d_offset(pgd,addr);+do{+BUG_ON(p4d_none(*p4d));++next=p4d_addr_end(addr,end);+vmemmap_pud_range(p4d,addr,next,walk);+}while(p4d++,addr=next,addr!=end);+}++staticvoidvmemmap_remap_range(unsignedlongstart,unsignedlongend,+structvmemmap_remap_walk*walk)+{+unsignedlongaddr=start;+unsignedlongnext;+pgd_t*pgd;++VM_BUG_ON(!IS_ALIGNED(start,PAGE_SIZE));+VM_BUG_ON(!IS_ALIGNED(end,PAGE_SIZE));++pgd=pgd_offset_k(addr);+do{+BUG_ON(pgd_none(*pgd));++next=pgd_addr_end(addr,end);+vmemmap_p4d_range(pgd,addr,next,walk);+}while(pgd++,addr=next,addr!=end);++flush_tlb_kernel_range(start,end);+}++/*+*Freeavmemmappage.Avmemmappagecanbeallocatedfromthememblock+*allocatororbuddyallocator.IfthePG_reservedflagisset,itmeans+*thatitallocatedfromthememblockallocator,justfreeitviathe+*free_bootmem_page().Otherwise,use__free_page().+*/+staticinlinevoidfree_vmemmap_page(structpage*page)+{+if(PageReserved(page))+free_bootmem_page(page);+else+__free_page(page);+}++/* Free a list of the vmemmap pages */+staticvoidfree_vmemmap_page_list(structlist_head*list)+{+structpage*page,*next;++list_for_each_entry_safe(page,next,list,lru){+list_del(&page->lru);+free_vmemmap_page(page);+}+}++staticvoidvmemmap_remap_pte(pte_t*pte,unsignedlongaddr,+structvmemmap_remap_walk*walk)+{+/*+*Remapthetailpagesasread-onlytocatchillegalwriteoperation+*tothetailpages.+*/+pgprot_tpgprot=PAGE_KERNEL_RO;+pte_tentry=mk_pte(walk->reuse_page,pgprot);+structpage*page=pte_page(*pte);++list_add(&page->lru,walk->vmemmap_pages);+set_pte_at(&init_mm,addr,pte,entry);+}++/**+*vmemmap_remap_free-remapthevmemmapvirtualaddressrange[@start,@end)+*tothepagewhich@reuseismapped,thenfreevmemmap+*pages.+*@start:startaddressofthevmemmapvirtualaddressrange.+*@end:endaddressofthevmemmapvirtualaddressrange.+*@reuse:reuseaddress.+*/+voidvmemmap_remap_free(unsignedlongstart,unsignedlongend,+unsignedlongreuse)+{+LIST_HEAD(vmemmap_pages);+structvmemmap_remap_walkwalk={+.remap_pte=vmemmap_remap_pte,+.reuse_addr=reuse,+.vmemmap_pages=&vmemmap_pages,+};++BUG_ON(start!=reuse+PAGE_SIZE);++vmemmap_remap_range(reuse,end,&walk);+free_vmemmap_page_list(&vmemmap_pages);+}/**Allocateablockofmemorytobeusedtobackthevirtualmemorymap
From: Muchun Song <hidden> Date: 2021-01-06 14:22:13
In the subsequent patch, we should allocate the vmemmap pages when
freeing HugeTLB pages. But update_and_free_page() is always called
with holding hugetlb_lock, so we cannot use GFP_KERNEL to allocate
vmemmap pages. However, we can defer the actual freeing in a kworker
to prevent from using GFP_ATOMIC to allocate the vmemmap pages.
The update_hpage_vmemmap_workfn() is where the call to allocate
vmemmmap pages will be inserted.
Signed-off-by: Muchun Song <redacted>
Reviewed-by: Mike Kravetz <redacted>
---
mm/hugetlb.c | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++--
mm/hugetlb_vmemmap.c | 12 ---------
mm/hugetlb_vmemmap.h | 17 ++++++++++++
3 files changed, 89 insertions(+), 14 deletions(-)
From: Muchun Song <hidden> Date: 2021-01-06 14:22:14
We should flush work when dissolving a HugeTLB page to make sure that
the HugeTLB page is freed to the buddy allocator. Because the caller
of dissolve_free_huge_pages() relies on this guarantee.
Signed-off-by: Muchun Song <redacted>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
---
mm/hugetlb.c | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)
@@ -1337,6 +1337,12 @@ static void update_hpage_vmemmap_workfn(struct work_struct *work)}staticDECLARE_WORK(hpage_update_work,update_hpage_vmemmap_workfn);+staticinlinevoidflush_hpage_update_work(structhstate*h)+{+if(free_vmemmap_pages_per_hpage(h))+flush_work(&hpage_update_work);+}+staticinlinevoid__update_and_free_page(structhstate*h,structpage*page){/* No need to allocate vmemmap pages */
@@ -1887,6 +1893,7 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,intdissolve_free_huge_page(structpage*page){intrc=-EBUSY;+structhstate*h=NULL;/* Not to disrupt normal path by vainly holding hugetlb_lock */if(!PageHuge(page))
@@ -1900,8 +1907,9 @@ int dissolve_free_huge_page(struct page *page)if(!page_count(page)){structpage*head=compound_head(page);-structhstate*h=page_hstate(head);intnid=page_to_nid(head);++h=page_hstate(head);if(h->free_huge_pages-h->resv_huge_pages==0)gotoout;
@@ -1915,6 +1923,14 @@ int dissolve_free_huge_page(struct page *page)}out:spin_unlock(&hugetlb_lock);++/*+*Weshouldflushworkbeforereturntomakesurethat+*theHugeTLBpageisfreedtothebuddy.+*/+if(!rc&&h)+flush_hpage_update_work(h);+returnrc;}
From: Muchun Song <hidden> Date: 2021-01-06 14:22:14
Because we reuse the first tail vmemmap page frame and remap it
with read-only, we cannot set the PageHWPosion on a tail page.
So we can use the head[4].private to record the real error page
index and set the raw error page PageHWPoison later.
Signed-off-by: Muchun Song <redacted>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
---
mm/hugetlb.c | 69 +++++++++++++++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 61 insertions(+), 8 deletions(-)
From: Muchun Song <hidden> Date: 2021-01-06 14:22:14
When we free a HugeTLB page to the buddy allocator, we should allocate the
vmemmap pages associated with it. We can do that in the __free_hugepage()
before freeing it to buddy.
Signed-off-by: Muchun Song <redacted>
---
include/linux/mm.h | 2 ++
mm/hugetlb.c | 2 ++
mm/hugetlb_vmemmap.c | 15 +++++++++++
mm/hugetlb_vmemmap.h | 5 ++++
mm/sparse-vmemmap.c | 76 +++++++++++++++++++++++++++++++++++++++++++++++++++-
5 files changed, 99 insertions(+), 1 deletion(-)
From: Muchun Song <hidden> Date: 2021-01-06 14:22:44
When we free a HugeTLB page whose vmemmap pages can be optimized,
it is freed to the buddy allocator through a kworker. And the ref
count of page is zero, so if we dissolve it before it is freed to
the buddy allocator. It can be freed again. In order to avoid
this, we introduce PageHugeInflight to indicate that the HugeTLB
page is already freed from hugepage pool but not freed to buddy
allocator yet. When we hit the inflight page, we just need to flush
the work.
Signed-off-by: Muchun Song <redacted>
---
mm/hugetlb.c | 38 +++++++++++++++++++++++++++++++++++++-
1 file changed, 37 insertions(+), 1 deletion(-)
@@ -1551,6 +1551,20 @@ Documentation/admin-guide/mm/hugetlbpage.rst. Format: size[KMG]+ hugetlb_free_vmemmap=+ [KNL] When CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is set,+ this controls freeing unused vmemmap pages associated+ with each HugeTLB page. When this option is enabled,+ we disable PMD/huge page mapping of vmemmap pages which+ increase page table pages. So if a user/sysadmin only+ uses a small number of HugeTLB pages (as a percentage+ of system memory), they could end up using more memory+ with hugetlb_free_vmemmap on as opposed to off.+ Format: { on | off (default) }++ on: enable the feature+ off: disable the feature+ hung_task_panic= [KNL] Should the hung task detector generate panics. Format: 0 | 1
@@ -145,6 +145,9 @@ default_hugepagesz will all result in 256 2M huge pages being allocated. Valid default huge page size is architecture dependent.+hugetlb_free_vmemmap+ When CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is set, this enables freeing+ unused vmemmap pages associated with each HugeTLB page. When multiple huge page sizes are supported, ``/proc/sys/vm/nr_hugepages`` indicates the current number of pre-allocated huge pages of the default size.
@@ -1557,7 +1558,8 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,{interr;-if(end-start<PAGES_PER_SECTION*sizeof(structpage))+if(is_hugetlb_free_vmemmap_enabled()||+end-start<PAGES_PER_SECTION*sizeof(structpage))err=vmemmap_populate_basepages(start,end,node,NULL);elseif(boot_cpu_has(X86_FEATURE_PSE))err=vmemmap_populate_hugepages(start,end,node,altmap);
@@ -1585,6 +1587,8 @@ void register_page_bootmem_memmap(unsigned long section_nr,pmd_t*pmd;unsignedintnr_pmd_pages;structpage*page;+boolbase_mapping=!boot_cpu_has(X86_FEATURE_PSE)||+is_hugetlb_free_vmemmap_enabled();for(;addr<end;addr=next){pte_t*pte=NULL;
@@ -1610,7 +1614,7 @@ void register_page_bootmem_memmap(unsigned long section_nr,}get_page_bootmem(section_nr,pud_page(*pud),MIX_SECTION_INFO);-if(!boot_cpu_has(X86_FEATURE_PSE)){+if(base_mapping){next=(addr+PAGE_SIZE)&PAGE_MASK;pmd=pmd_offset(pud,addr);if(pmd_none(*pmd))
From: Muchun Song <hidden> Date: 2021-01-06 14:23:15
All the infrastructure is ready, so we introduce nr_free_vmemmap_pages
field in the hstate to indicate how many vmemmap pages associated with
a HugeTLB page that can be freed to buddy allocator. And initialize it
in the hugetlb_vmemmap_init(). This patch is actual enablement of the
feature.
Signed-off-by: Muchun Song <redacted>
Acked-by: Mike Kravetz <redacted>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
---
include/linux/hugetlb.h | 3 +++
mm/hugetlb.c | 1 +
mm/hugetlb_vmemmap.c | 25 +++++++++++++++++++++++++
mm/hugetlb_vmemmap.h | 10 ++++++----
4 files changed, 35 insertions(+), 4 deletions(-)
From: Muchun Song <hidden> Date: 2021-01-06 14:23:16
For HugeTLB page, there are more metadata to save in the struct page.
But the head struct page cannot meet our needs, so we have to abuse
other tail struct page to store the metadata. In order to avoid
conflicts caused by subsequent use of more tail struct pages, we can
gather these discrete indexes of tail struct page. In this case, it
will be easier to add a new tail page index later.
There are only (RESERVE_VMEMMAP_SIZE / sizeof(struct page)) struct
page structs that can be used when CONFIG_HUGETLB_PAGE_FREE_VMEMMAP,
so add a BUILD_BUG_ON to catch invalid usage of the tail struct page.
Signed-off-by: Muchun Song <redacted>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
---
include/linux/hugetlb.h | 14 ++++++++++++++
include/linux/hugetlb_cgroup.h | 15 +++++++++------
mm/hugetlb.c | 25 ++++++++++++-------------
mm/hugetlb_vmemmap.c | 8 ++++++++
4 files changed, 43 insertions(+), 19 deletions(-)
@@ -1514,20 +1513,20 @@ struct hstate *size_to_hstate(unsigned long size)boolpage_huge_active(structpage*page){VM_BUG_ON_PAGE(!PageHuge(page),page);-returnPageHead(page)&&PagePrivate(&page[1]);+returnPageHead(page)&&PagePrivate(&page[SUBPAGE_INDEX_ACTIVE]);}/* never called for tail page */staticvoidset_page_huge_active(structpage*page){VM_BUG_ON_PAGE(!PageHeadHuge(page),page);-SetPagePrivate(&page[1]);+SetPagePrivate(&page[SUBPAGE_INDEX_ACTIVE]);}staticvoidclear_page_huge_active(structpage*page){VM_BUG_ON_PAGE(!PageHeadHuge(page),page);-ClearPagePrivate(&page[1]);+ClearPagePrivate(&page[SUBPAGE_INDEX_ACTIVE]);}/*
From: Muchun Song <hidden> Date: 2021-01-06 14:23:16
We cannot optimize if a "struct page" crosses page boundaries. If
it is true, we can optimize the code with the help of a compiler.
When free_vmemmap_pages_per_hpage() returns zero, most functions are
optimized by the compiler.
Signed-off-by: Muchun Song <redacted>
---
include/linux/hugetlb.h | 3 ++-
mm/hugetlb_vmemmap.c | 7 +++++++
mm/hugetlb_vmemmap.h | 5 +++--
3 files changed, 12 insertions(+), 3 deletions(-)
From: Oscar Salvador <osalvador@suse.de> Date: 2021-01-12 08:06:22
On Wed, Jan 06, 2021 at 10:19:22PM +0800, Muchun Song wrote:
Every HugeTLB has more than one struct page structure. We __know__ that
we only use the first 4(HUGETLB_CGROUP_MIN_ORDER) struct page structures
to store metadata associated with each HugeTLB.
There are a lot of struct page structures associated with each HugeTLB
page. For tail pages, the value of compound_head is the same. So we can
reuse first page of tail page structures. We map the virtual addresses
of the remaining pages of tail page structures to the first tail page
struct, and then free these page frames. Therefore, we need to reserve
two pages as vmemmap areas.
When we allocate a HugeTLB page from the buddy, we can free some vmemmap
pages associated with each HugeTLB page. It is more appropriate to do it
in the prep_new_huge_page().
The free_vmemmap_pages_per_hpage(), which indicates how many vmemmap
pages associated with a HugeTLB page can be freed, returns zero for
now, which means the feature is disabled. We will enable it once all
the infrastructure is there.
Signed-off-by: Muchun Song <redacted>
My memory may betray me after vacation, so bear with me.
+/*
+ * Any memory allocated via the memblock allocator and not via the
+ * buddy will be marked reserved already in the memmap. For those
+ * pages, we can call this function to free it to buddy allocator.
+ */
+static inline void free_bootmem_page(struct page *page)
+{
+ unsigned long magic = (unsigned long)page->freelist;
+
+ /*
+ * The reserve_bootmem_region sets the reserved flag on bootmem
+ * pages.
+ */
+ VM_WARN_ON_PAGE(page_ref_count(page) != 2, page);
I have been thinking about this some more.
And while I think that this macro might have its room somewhere, I do not
think this is the case.
Here, if we see that page's refcount differs from 2 it means that we had an
earlier corruption.
Now, as a person that has dealt with debugging memory corruptions, I think it
is of no use to proceed further if such corruption happened, as this can lead
to problems somewhere else that can manifest in funny ways, and you will find
yourself scratching your head and trying to work out what happened.
I am aware that this is not the root of the problem here, as someone might have
had to decrease the refcount, but I would definitely change this to its
VM_BUG_ON_* variant.
+/**
+ * vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
+ * to the page which @reuse is mapped, then free vmemmap
+ * pages.
+ * @start: start address of the vmemmap virtual address range.
+ * @end: end address of the vmemmap virtual address range.
+ * @reuse: reuse address.
+ */
+void vmemmap_remap_free(unsigned long start, unsigned long end,
+ unsigned long reuse)
+{
+ LIST_HEAD(vmemmap_pages);
+ struct vmemmap_remap_walk walk = {
+ .remap_pte = vmemmap_remap_pte,
+ .reuse_addr = reuse,
+ .vmemmap_pages = &vmemmap_pages,
+ };
+
+ BUG_ON(start != reuse + PAGE_SIZE);
It seems a bit odd to only pass "start" for the BUG_ON.
Also, I kind of dislike the "addr += PAGE_SIZE" in vmemmap_pte_range.
I wonder if adding a ".remap_start_addr" would make more sense.
And adding it here with the vmemmap_remap_walk init.
--
Oscar Salvador
SUSE L3
From: Muchun Song <hidden> Date: 2021-01-12 11:35:24
On Tue, Jan 12, 2021 at 4:05 PM Oscar Salvador [off-list ref] wrote:
On Wed, Jan 06, 2021 at 10:19:22PM +0800, Muchun Song wrote:
quoted
Every HugeTLB has more than one struct page structure. We __know__ that
we only use the first 4(HUGETLB_CGROUP_MIN_ORDER) struct page structures
to store metadata associated with each HugeTLB.
There are a lot of struct page structures associated with each HugeTLB
page. For tail pages, the value of compound_head is the same. So we can
reuse first page of tail page structures. We map the virtual addresses
of the remaining pages of tail page structures to the first tail page
struct, and then free these page frames. Therefore, we need to reserve
two pages as vmemmap areas.
When we allocate a HugeTLB page from the buddy, we can free some vmemmap
pages associated with each HugeTLB page. It is more appropriate to do it
in the prep_new_huge_page().
The free_vmemmap_pages_per_hpage(), which indicates how many vmemmap
pages associated with a HugeTLB page can be freed, returns zero for
now, which means the feature is disabled. We will enable it once all
the infrastructure is there.
Signed-off-by: Muchun Song <redacted>
My memory may betray me after vacation, so bear with me.
Welcome back. :)
quoted
+/*
+ * Any memory allocated via the memblock allocator and not via the
+ * buddy will be marked reserved already in the memmap. For those
+ * pages, we can call this function to free it to buddy allocator.
+ */
+static inline void free_bootmem_page(struct page *page)
+{
+ unsigned long magic = (unsigned long)page->freelist;
+
+ /*
+ * The reserve_bootmem_region sets the reserved flag on bootmem
+ * pages.
+ */
+ VM_WARN_ON_PAGE(page_ref_count(page) != 2, page);
I have been thinking about this some more.
And while I think that this macro might have its room somewhere, I do not
think this is the case.
Here, if we see that page's refcount differs from 2 it means that we had an
earlier corruption.
Now, as a person that has dealt with debugging memory corruptions, I think it
is of no use to proceed further if such corruption happened, as this can lead
to problems somewhere else that can manifest in funny ways, and you will find
yourself scratching your head and trying to work out what happened.
I am aware that this is not the root of the problem here, as someone might have
had to decrease the refcount, but I would definitely change this to its
VM_BUG_ON_* variant.
+/**
+ * vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
+ * to the page which @reuse is mapped, then free vmemmap
+ * pages.
+ * @start: start address of the vmemmap virtual address range.
+ * @end: end address of the vmemmap virtual address range.
+ * @reuse: reuse address.
+ */
+void vmemmap_remap_free(unsigned long start, unsigned long end,
+ unsigned long reuse)
+{
+ LIST_HEAD(vmemmap_pages);
+ struct vmemmap_remap_walk walk = {
+ .remap_pte = vmemmap_remap_pte,
+ .reuse_addr = reuse,
+ .vmemmap_pages = &vmemmap_pages,
+ };
+
+ BUG_ON(start != reuse + PAGE_SIZE);
It seems a bit odd to only pass "start" for the BUG_ON.
Also, I kind of dislike the "addr += PAGE_SIZE" in vmemmap_pte_range.
I wonder if adding a ".remap_start_addr" would make more sense.
And adding it here with the vmemmap_remap_walk init.
How about introducing a new function which aims to get the reuse
page? In this case, we can drop the BUG_ON() and "addr += PAGE_SIZE"
which is in vmemmap_pte_range. The vmemmap_remap_range only
does the remapping.
From: Oscar Salvador <osalvador@suse.de> Date: 2021-01-13 09:21:34
On Tue, Jan 12, 2021 at 07:33:33PM +0800, Muchun Song wrote:
quoted
It seems a bit odd to only pass "start" for the BUG_ON.
Also, I kind of dislike the "addr += PAGE_SIZE" in vmemmap_pte_range.
I wonder if adding a ".remap_start_addr" would make more sense.
And adding it here with the vmemmap_remap_walk init.
How about introducing a new function which aims to get the reuse
page? In this case, we can drop the BUG_ON() and "addr += PAGE_SIZE"
which is in vmemmap_pte_range. The vmemmap_remap_range only
does the remapping.
How would that look?
It might be good, dunno, but the point is, we should try to make the rules as
simple as possible, dropping weird assumptions.
Callers of vmemmap_remap_free should know three things:
- Range to be remapped
- Addr to remap to
- Current implemantion needs addr to be remap to to be part of the complete
range
right?
--
Oscar Salvador
SUSE L3
From: Mike Kravetz <hidden> Date: 2021-01-14 01:55:49
On 1/13/21 1:20 AM, Oscar Salvador wrote:
On Tue, Jan 12, 2021 at 07:33:33PM +0800, Muchun Song wrote:
quoted
quoted
It seems a bit odd to only pass "start" for the BUG_ON.
Also, I kind of dislike the "addr += PAGE_SIZE" in vmemmap_pte_range.
I wonder if adding a ".remap_start_addr" would make more sense.
And adding it here with the vmemmap_remap_walk init.
How about introducing a new function which aims to get the reuse
page? In this case, we can drop the BUG_ON() and "addr += PAGE_SIZE"
which is in vmemmap_pte_range. The vmemmap_remap_range only
does the remapping.
How would that look?
It might be good, dunno, but the point is, we should try to make the rules as
simple as possible, dropping weird assumptions.
Callers of vmemmap_remap_free should know three things:
- Range to be remapped
- Addr to remap to
- Current implemantion needs addr to be remap to to be part of the complete
range
right?
And, current implementation needs must have remap addr be the first in the
complete range. This is just because of the way the page tables are walked
for remapping. The remap/reuse page must be found first so that the following
pages can be remapped to it.
That implementation seems to be the 'most efficient' for hugetlb pages where
we want vmemmap pages n+3 and beyond mapped to n+2.
In a more general purpose vmemmap_remap_free implementation, the reuse/remap
address would not necessarily need to be related to the range. However, this
would require a separate page table walk/validation for the reuse address
independent of the range. This may be what Muchun was proposing for 'a new
function which aims to get the reuse page'.
IMO, the decision on how to implement depends on the intended use case.
- If this is going to be hugetlb only (or perhaps generic huge page only)
functionality, then I am OK with an efficient implementation that has
some restrictions.
- If we see this being used for more general purpose remapping, then we
should go with a more general purpose implementation.
Again, just my opinion.
--
Mike Kravetz
From: Mike Kravetz <hidden> Date: 2021-01-14 02:04:47
On 1/6/21 6:19 AM, Muchun Song wrote:
Very similar to VM_WARN_ON_ONCE_PAGE and VM_BUG_ON_PAGE, add
VM_WARN_ON_PAGE macro.
Signed-off-by: Muchun Song <redacted>
---
include/linux/mmdebug.h | 8 ++++++++
1 file changed, 8 insertions(+)
I was going to question the use/need for this macro in the following
patch. Looks like Oscar has already done that, and free_bootmem_page
will now use VM_BUG_ON_PAGE. So, this patch can be dropped.
--
Mike Kravetz
From: Mike Kravetz <hidden> Date: 2021-01-14 02:18:52
On 1/6/21 6:19 AM, Muchun Song wrote:
For HugeTLB page, there are more metadata to save in the struct page.
But the head struct page cannot meet our needs, so we have to abuse
other tail struct page to store the metadata. In order to avoid
conflicts caused by subsequent use of more tail struct pages, we can
gather these discrete indexes of tail struct page. In this case, it
will be easier to add a new tail page index later.
There are only (RESERVE_VMEMMAP_SIZE / sizeof(struct page)) struct
page structs that can be used when CONFIG_HUGETLB_PAGE_FREE_VMEMMAP,
so add a BUILD_BUG_ON to catch invalid usage of the tail struct page.
Signed-off-by: Muchun Song <redacted>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
---
include/linux/hugetlb.h | 14 ++++++++++++++
include/linux/hugetlb_cgroup.h | 15 +++++++++------
mm/hugetlb.c | 25 ++++++++++++-------------
mm/hugetlb_vmemmap.c | 8 ++++++++
4 files changed, 43 insertions(+), 19 deletions(-)
My apologies! I did not get to this patch in previous versions of the
series. My "RFC create hugetlb flags to consolidate state" was done
before I even noticed your efforts here.
At least we agree the metadata could be better organized. :)
IMO, using page.private of the head page to consolidate flags will be
easier to manage. So, I would like to use that.
The BUILD_BUG_ON in this patch makes sense.
--
Mike Kravetz
From: Muchun Song <hidden> Date: 2021-01-14 02:48:29
On Thu, Jan 14, 2021 at 8:18 AM Mike Kravetz [off-list ref] wrote:
On 1/6/21 6:19 AM, Muchun Song wrote:
quoted
For HugeTLB page, there are more metadata to save in the struct page.
But the head struct page cannot meet our needs, so we have to abuse
other tail struct page to store the metadata. In order to avoid
conflicts caused by subsequent use of more tail struct pages, we can
gather these discrete indexes of tail struct page. In this case, it
will be easier to add a new tail page index later.
There are only (RESERVE_VMEMMAP_SIZE / sizeof(struct page)) struct
page structs that can be used when CONFIG_HUGETLB_PAGE_FREE_VMEMMAP,
so add a BUILD_BUG_ON to catch invalid usage of the tail struct page.
Signed-off-by: Muchun Song <redacted>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
---
include/linux/hugetlb.h | 14 ++++++++++++++
include/linux/hugetlb_cgroup.h | 15 +++++++++------
mm/hugetlb.c | 25 ++++++++++++-------------
mm/hugetlb_vmemmap.c | 8 ++++++++
4 files changed, 43 insertions(+), 19 deletions(-)
My apologies! I did not get to this patch in previous versions of the
series. My "RFC create hugetlb flags to consolidate state" was done
before I even noticed your efforts here.
At least we agree the metadata could be better organized. :)
IMO, using page.private of the head page to consolidate flags will be
easier to manage. So, I would like to use that.
Agree. Using page.private to manage the page flag is a good
choice. When your RFC patch is applied, I will rebase it and
rework this patch.
The BUILD_BUG_ON in this patch makes sense.
--
Mike Kravetz
From: Muchun Song <hidden> Date: 2021-01-14 02:52:28
On Thu, Jan 14, 2021 at 6:31 AM Mike Kravetz [off-list ref] wrote:
On 1/6/21 6:19 AM, Muchun Song wrote:
quoted
Very similar to VM_WARN_ON_ONCE_PAGE and VM_BUG_ON_PAGE, add
VM_WARN_ON_PAGE macro.
Signed-off-by: Muchun Song <redacted>
---
include/linux/mmdebug.h | 8 ++++++++
1 file changed, 8 insertions(+)
I was going to question the use/need for this macro in the following
patch. Looks like Oscar has already done that, and free_bootmem_page
will now use VM_BUG_ON_PAGE. So, this patch can be dropped.
In the previous version, Oscar suggested I use WARN_ON instead of
BUG_ON. But he realised we actually should use BUG_ON in this
version. So I will drop this in the next version. Thanks.
From: Muchun Song <hidden> Date: 2021-01-14 10:56:10
On Thu, Jan 14, 2021 at 7:27 AM Mike Kravetz [off-list ref] wrote:
On 1/13/21 1:20 AM, Oscar Salvador wrote:
quoted
On Tue, Jan 12, 2021 at 07:33:33PM +0800, Muchun Song wrote:
quoted
quoted
It seems a bit odd to only pass "start" for the BUG_ON.
Also, I kind of dislike the "addr += PAGE_SIZE" in vmemmap_pte_range.
I wonder if adding a ".remap_start_addr" would make more sense.
And adding it here with the vmemmap_remap_walk init.
How about introducing a new function which aims to get the reuse
page? In this case, we can drop the BUG_ON() and "addr += PAGE_SIZE"
which is in vmemmap_pte_range. The vmemmap_remap_range only
does the remapping.
How would that look?
It might be good, dunno, but the point is, we should try to make the rules as
simple as possible, dropping weird assumptions.
Callers of vmemmap_remap_free should know three things:
- Range to be remapped
- Addr to remap to
- Current implemantion needs addr to be remap to to be part of the complete
range
right?
And, current implementation needs must have remap addr be the first in the
complete range. This is just because of the way the page tables are walked
for remapping. The remap/reuse page must be found first so that the following
pages can be remapped to it.
You are right.
That implementation seems to be the 'most efficient' for hugetlb pages where
we want vmemmap pages n+3 and beyond mapped to n+2.
In a more general purpose vmemmap_remap_free implementation, the reuse/remap
address would not necessarily need to be related to the range. However, this
would require a separate page table walk/validation for the reuse address
independent of the range. This may be what Muchun was proposing for 'a new
function which aims to get the reuse page'.
Agree.
IMO, the decision on how to implement depends on the intended use case.
- If this is going to be hugetlb only (or perhaps generic huge page only)
functionality, then I am OK with an efficient implementation that has
some restrictions.
- If we see this being used for more general purpose remapping, then we
should go with a more general purpose implementation.
I think this approach may be only suitable for generic huge page only.
So we can implement it only for huge page.
Hi Oscar,
What's your opinion about this?
From: Muchun Song <hidden> Date: 2021-01-14 10:58:58
On Tue, Jan 12, 2021 at 4:05 PM Oscar Salvador [off-list ref] wrote:
On Wed, Jan 06, 2021 at 10:19:22PM +0800, Muchun Song wrote:
quoted
Every HugeTLB has more than one struct page structure. We __know__ that
we only use the first 4(HUGETLB_CGROUP_MIN_ORDER) struct page structures
to store metadata associated with each HugeTLB.
There are a lot of struct page structures associated with each HugeTLB
page. For tail pages, the value of compound_head is the same. So we can
reuse first page of tail page structures. We map the virtual addresses
of the remaining pages of tail page structures to the first tail page
struct, and then free these page frames. Therefore, we need to reserve
two pages as vmemmap areas.
When we allocate a HugeTLB page from the buddy, we can free some vmemmap
pages associated with each HugeTLB page. It is more appropriate to do it
in the prep_new_huge_page().
The free_vmemmap_pages_per_hpage(), which indicates how many vmemmap
pages associated with a HugeTLB page can be freed, returns zero for
now, which means the feature is disabled. We will enable it once all
the infrastructure is there.
Signed-off-by: Muchun Song <redacted>
My memory may betray me after vacation, so bear with me.
quoted
+/*
+ * Any memory allocated via the memblock allocator and not via the
+ * buddy will be marked reserved already in the memmap. For those
+ * pages, we can call this function to free it to buddy allocator.
+ */
+static inline void free_bootmem_page(struct page *page)
+{
+ unsigned long magic = (unsigned long)page->freelist;
+
+ /*
+ * The reserve_bootmem_region sets the reserved flag on bootmem
+ * pages.
+ */
+ VM_WARN_ON_PAGE(page_ref_count(page) != 2, page);
I have been thinking about this some more.
And while I think that this macro might have its room somewhere, I do not
think this is the case.
Here, if we see that page's refcount differs from 2 it means that we had an
earlier corruption.
Now, as a person that has dealt with debugging memory corruptions, I think it
is of no use to proceed further if such corruption happened, as this can lead
to problems somewhere else that can manifest in funny ways, and you will find
yourself scratching your head and trying to work out what happened.
I am aware that this is not the root of the problem here, as someone might have
had to decrease the refcount, but I would definitely change this to its
VM_BUG_ON_* variant.
+/**
+ * vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
+ * to the page which @reuse is mapped, then free vmemmap
+ * pages.
+ * @start: start address of the vmemmap virtual address range.
+ * @end: end address of the vmemmap virtual address range.
+ * @reuse: reuse address.
+ */
+void vmemmap_remap_free(unsigned long start, unsigned long end,
+ unsigned long reuse)
+{
+ LIST_HEAD(vmemmap_pages);
+ struct vmemmap_remap_walk walk = {
+ .remap_pte = vmemmap_remap_pte,
+ .reuse_addr = reuse,
+ .vmemmap_pages = &vmemmap_pages,
+ };
+
+ BUG_ON(start != reuse + PAGE_SIZE);
It seems a bit odd to only pass "start" for the BUG_ON.
Also, I kind of dislike the "addr += PAGE_SIZE" in vmemmap_pte_range.
I wonder if adding a ".remap_start_addr" would make more sense.
And adding it here with the vmemmap_remap_walk init.
How would vmemmap_pte_range look? If we introduce
vmemmap_remap_walk, "addr += PAGE_SIZE" can drop?
From: Oscar Salvador <osalvador@suse.de> Date: 2021-01-14 11:53:52
On Thu, Jan 14, 2021 at 06:54:30PM +0800, Muchun Song wrote:
I think this approach may be only suitable for generic huge page only.
So we can implement it only for huge page.
Hi Oscar,
What's your opinion about this?
I tried something like:
static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
unsigned long end,
struct vmemmap_remap_walk *walk)
{
pte_t *pte;
pte = pte_offset_kernel(pmd, addr);
if (!walk->reuse_page) {
BUG_ON(pte_none(*pte));
walk->reuse_page = pte_page(*pte++);
addr = walk->remap_start;
}
for (; addr != end; addr += PAGE_SIZE, pte++) {
BUG_ON(pte_none(*pte));
walk->remap_pte(pte, addr, walk);
}
}
void vmemmap_remap_free(unsigned long start, unsigned long end,
unsigned long reuse)
{
LIST_HEAD(vmemmap_pages);
struct vmemmap_remap_walk walk = {
.remap_pte = vmemmap_remap_pte,
.reuse_addr = reuse,
.remap_start = start,
.vmemmap_pages = &vmemmap_pages,
};
BUG_ON(start != reuse + PAGE_SIZE);
vmemmap_remap_range(reuse, end, &walk);
free_vmemmap_page_list(&vmemmap_pages);
}
but it might overcomplicate things and I am not sure it is any better.
So I am fine with keeping it as is.
Should another user come in the future, we can always revisit.
Maybe just add a little comment in vmemmap_pte_range(), explaining while we
are "+= PAGE_SIZE" for address and I would like to see a comment in
vmemmap_remap_free why the BUG_ON and more important what it is checking.
--
Oscar Salvador
SUSE L3
From: Muchun Song <hidden> Date: 2021-01-14 13:06:42
On Thu, Jan 14, 2021 at 7:52 PM Oscar Salvador [off-list ref] wrote:
On Thu, Jan 14, 2021 at 06:54:30PM +0800, Muchun Song wrote:
quoted
I think this approach may be only suitable for generic huge page only.
So we can implement it only for huge page.
Hi Oscar,
What's your opinion about this?
I tried something like:
static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
unsigned long end,
struct vmemmap_remap_walk *walk)
{
pte_t *pte;
pte = pte_offset_kernel(pmd, addr);
if (!walk->reuse_page) {
BUG_ON(pte_none(*pte));
walk->reuse_page = pte_page(*pte++);
addr = walk->remap_start;
}
for (; addr != end; addr += PAGE_SIZE, pte++) {
BUG_ON(pte_none(*pte));
walk->remap_pte(pte, addr, walk);
}
}
void vmemmap_remap_free(unsigned long start, unsigned long end,
unsigned long reuse)
{
LIST_HEAD(vmemmap_pages);
struct vmemmap_remap_walk walk = {
.remap_pte = vmemmap_remap_pte,
.reuse_addr = reuse,
.remap_start = start,
.vmemmap_pages = &vmemmap_pages,
};
BUG_ON(start != reuse + PAGE_SIZE);
vmemmap_remap_range(reuse, end, &walk);
free_vmemmap_page_list(&vmemmap_pages);
}
but it might overcomplicate things and I am not sure it is any better.
So I am fine with keeping it as is.
Should another user come in the future, we can always revisit.
Maybe just add a little comment in vmemmap_pte_range(), explaining while we
are "+= PAGE_SIZE" for address and I would like to see a comment in
vmemmap_remap_free why the BUG_ON and more important what it is checking.
OK, I will add some comments to explain why we do this in
vmemmap_remap_free and vmemmap_remap_free. Thanks.