On 20.05.21 13:54, Anshuman Khandual wrote:

On 5/19/21 5:33 PM, David Hildenbrand wrote:

quoted

On 19.05.21 13:45, Anshuman Khandual wrote:

quoted

On 5/18/21 2:48 PM, Muchun Song wrote:

quoted

The preparation of supporting freeing vmemmap associated with each
HugeTLB page is ready, so we can support this feature for arm64.

Signed-off-by: Muchun Song <redacted>
---
   arch/arm64/mm/mmu.c | 5 +++++
   fs/Kconfig          | 2 +-
   2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 5d37e461c41f..967b01ce468d 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c

@@ -23,6 +23,7 @@
   #include <linux/mm.h>
   #include <linux/vmalloc.h>
   #include <linux/set_memory.h>
+#include <linux/hugetlb.h>
     #include <asm/barrier.h>
   #include <asm/cputype.h>

@@ -1134,6 +1135,10 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
       pmd_t *pmdp;
         WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
+
+    if (is_hugetlb_free_vmemmap_enabled() && !altmap)
+        return vmemmap_populate_basepages(start, end, node, altmap);

Not considering the fact that this will force the kernel to have only
base page size mapping for vmemmap (unless altmap is also requested)
which might reduce the performance, it also enables vmemmap mapping to
be teared down or build up at runtime which could potentially collide
with other kernel page table walkers like ptdump or memory hotremove
operation ! How those possible collisions are protected right now ?

Hi Anshuman,

Memory hotremove is not an issue IIRC. At the time memory is removed, all huge pages either have been migrated away or dissolved; the vmemmap is stable.

But what happens when a hot remove section's vmemmap area (which is being
teared down) is nearby another vmemmap area which is either created or
being destroyed for HugeTLB alloc/free purpose. As you mentioned HugeTLB
pages inside the hot remove section might be safe. But what about other
HugeTLB areas whose vmemmap area shares page table entries with vmemmap
entries for a section being hot removed ? Massive HugeTLB alloc/use/free
test cycle using memory just adjacent to a memory hotplug area, which is
always added and removed periodically, should be able to expose this problem.

IIUC unlike vmalloc(), vmemap mapping areas in the kernel page table were
always constant unless there are hotplug add or remove operations which
are protected with a hotplug lock. Now with this change, we could have
simultaneous walking and add or remove of the vmemap areas without any
synchronization. Is not this problematic ?

On arm64 memory hot remove operation empties free portions of the vmemmap
table after clearing them. Hence all concurrent walkers (hugetlb_vmemmap,
hot remove, ptdump etc) need to be synchronized against hot remove.

 From arch/arm64/mm/mmu.c

void vmemmap_free(unsigned long start, unsigned long end,
                 struct vmem_altmap *altmap)
{
#ifdef CONFIG_MEMORY_HOTPLUG
         WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));

         unmap_hotplug_range(start, end, true, altmap);
         free_empty_tables(start, end, VMEMMAP_START, VMEMMAP_END);
#endif
}

You are right, however, AFAIR

1) We always populate base pages, meaning we only modify PTEs and not 
actually add/remove page tables when creating/destroying a hugetlb page. 
Page table walkers should be fine and not suddenly run into a 
use-after-free.

2) For pfn_to_page() users to never fault, we have to do an atomic 
exchange of PTES, meaning, someone traversing a page table looking for 
pte_none() entries (like free_empty_tables() in your example) should 
never get a false positive.

Makes sense, or am I missing something?

quoted

vmemmap access (accessing the memmap via a virtual address) itself is not an issue. Manually walking (vmemmap) page tables might behave

Right.

differently, not sure if ptdump would require any synchronization.

Dumping an wrong value is probably okay but crashing because a page table
entry is being freed after ptdump acquired the pointer is bad. On arm64,
ptdump() is protected against hotremove via [get|put]_online_mems().

Okay, and as the feature in question only exchanges PTEs, we should be 
fine.



-- 
Thanks,

David / dhildenb


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