[PATCH v16 0/7] MADV_FREE support

Linux doesn't have an ability to free pages lazy while other OS
already have been supported that named by madvise(MADV_FREE).

The gain is clear that kernel can discard freed pages rather than
swapping out or OOM if memory pressure happens.

Without memory pressure, freed pages would be reused by userspace
without another additional overhead(ex, page fault + allocation
+ zeroing).

How to work is following as.

When madvise syscall is called, VM clears dirty bit of ptes of
the range. If memory pressure happens, VM checks dirty bit of
page table and if it found still "clean", it means it's a
"lazyfree pages" so VM could discard the page instead of swapping out.
Once there was store operation for the page before VM peek a page
to reclaim, dirty bit is set so VM can swap out the page instead of
discarding.

Firstly, heavy users would be general allocators(ex, jemalloc,
tcmalloc and hope glibc supports it) and jemalloc/tcmalloc already
have supported the feature for other OS(ex, FreeBSD)

barrios@blaptop:~/benchmark/ebizzy$ lscpu
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                4
On-line CPU(s) list:   0-3
Thread(s) per core:    2
Core(s) per socket:    2
Socket(s):             1
NUMA node(s):          1
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 42
Stepping:              7
CPU MHz:               2801.000
BogoMIPS:              5581.64
Virtualization:        VT-x
L1d cache:             32K
L1i cache:             32K
L2 cache:              256K
L3 cache:              4096K
NUMA node0 CPU(s):     0-3

ebizzy benchmark(./ebizzy -S 10 -n 512)

 vanilla-jemalloc               MADV_free-jemalloc

1 thread
records:  10              records:  10
avg:      7682.10         avg:      15306.10
std:      62.35(0.81%)    std:      347.99(2.27%)
max:      7770.00         max:      15622.00
min:      7598.00         min:      14772.00

2 thread
records:  10              records:  10
avg:      12747.50        avg:      24171.00
std:      792.06(6.21%)   std:      895.18(3.70%)
max:      13337.00        max:      26023.00
min:      10535.00        min:      23152.00

4 thread
records:  10              records:  10
avg:      16474.60        avg:      33717.90
std:      1496.45(9.08%)  std:      2008.97(5.96%)
max:      17877.00        max:      35958.00
min:      12224.00        min:      29565.00

8 thread
records:  10              records:  10
avg:      16778.50        avg:      33308.10
std:      825.53(4.92%)   std:      1668.30(5.01%)
max:      17543.00        max:      36010.00
min:      14576.00        min:      29577.00

16 thread
records:  10              records:  10
avg:      20614.40        avg:      35516.30
std:      602.95(2.92%)   std:      1283.65(3.61%)
max:      21753.00        max:      37178.00
min:      19605.00        min:      33217.00

32 thread
records:  10              records:  10
avg:      22771.70        avg:      36018.50
std:      598.94(2.63%)   std:      1046.76(2.91%)
max:      24035.00        max:      37266.00
min:      22108.00        min:      34149.00

In summary, MADV_FREE is about 2 time faster than MADV_DONTNEED.

Cc: Michael Kerrisk <redacted>
Cc: Linux API <redacted>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: KOSAKI Motohiro <redacted>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Jason Evans <redacted>
Acked-by: Kirill A. Shutemov <redacted>
Acked-by: Zhang Yanfei <redacted>
Acked-by: Rik van Riel <redacted>
Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 include/linux/rmap.h                   |   9 ++-
 include/linux/vm_event_item.h          |   1 +
 include/uapi/asm-generic/mman-common.h |   1 +
 mm/madvise.c                           | 140 +++++++++++++++++++++++++++++++++
 mm/rmap.c                              |  42 +++++++++-
 mm/vmscan.c                            |  40 ++++++++--
 mm/vmstat.c                            |   1 +
 7 files changed, 222 insertions(+), 12 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index c0c2bce6b0b7..14ec4ece14a5 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h

@@ -75,6 +75,7 @@ enum ttu_flags {
        TTU_UNMAP = 1,                  /* unmap mode */
        TTU_MIGRATION = 2,              /* migration mode */
        TTU_MUNLOCK = 4,                /* munlock mode */
+       TTU_FREE = 8,                   /* free mode */

        TTU_IGNORE_MLOCK = (1 << 8),    /* ignore mlock */
        TTU_IGNORE_ACCESS = (1 << 9),   /* don't age */

@@ -181,7 +182,8 @@ static inline void page_dup_rmap(struct page *page)
  * Called from mm/vmscan.c to handle paging out
  */
 int page_referenced(struct page *, int is_locked,
-                       struct mem_cgroup *memcg, unsigned long *vm_flags);
+                       struct mem_cgroup *memcg, unsigned long *vm_flags,
+                       int *is_dirty);

 #define TTU_ACTION(x) ((x) & TTU_ACTION_MASK)

@@ -260,9 +262,12 @@ int rmap_walk(struct page *page, struct rmap_walk_control *rwc);

 static inline int page_referenced(struct page *page, int is_locked,
                                  struct mem_cgroup *memcg,
-                                 unsigned long *vm_flags)
+                                 unsigned long *vm_flags,
+                                 int *is_pte_dirty)
 {
        *vm_flags = 0;
+       if (is_pte_dirty)
+               *is_pte_dirty = 0;
        return 0;
 }

diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index ced92345c963..e2d3fb1e9814 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h

@@ -25,6 +25,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
                FOR_ALL_ZONES(PGALLOC),
                PGFREE, PGACTIVATE, PGDEACTIVATE,
                PGFAULT, PGMAJFAULT,
+               PGLAZYFREED,
                FOR_ALL_ZONES(PGREFILL),
                FOR_ALL_ZONES(PGSTEAL_KSWAPD),
                FOR_ALL_ZONES(PGSTEAL_DIRECT),

diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h
index ddc3b36f1046..7a94102b7a02 100644
--- a/include/uapi/asm-generic/mman-common.h
+++ b/include/uapi/asm-generic/mman-common.h

@@ -34,6 +34,7 @@
 #define MADV_SEQUENTIAL        2               /* expect sequential page references */
 #define MADV_WILLNEED  3               /* will need these pages */
 #define MADV_DONTNEED  4               /* don't need these pages */
+#define MADV_FREE      5               /* free pages only if memory pressure */

 /* common parameters: try to keep these consistent across architectures */
 #define MADV_REMOVE    9               /* remove these pages & resources */

diff --git a/mm/madvise.c b/mm/madvise.c
index 0938b30da4ab..a21584235bb6 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c

@@ -19,6 +19,14 @@
 #include <linux/blkdev.h>
 #include <linux/swap.h>
 #include <linux/swapops.h>
+#include <linux/mmu_notifier.h>
+
+#include <asm/tlb.h>
+
+struct madvise_free_private {
+       struct vm_area_struct *vma;
+       struct mmu_gather *tlb;
+};

 /*
  * Any behaviour which results in changes to the vma->vm_flags needs to

@@ -31,6 +39,7 @@ static int madvise_need_mmap_write(int behavior)
        case MADV_REMOVE:
        case MADV_WILLNEED:
        case MADV_DONTNEED:
+       case MADV_FREE:
                return 0;
        default:
                /* be safe, default to 1. list exceptions explicitly */

@@ -251,6 +260,128 @@ static long madvise_willneed(struct vm_area_struct *vma,
        return 0;
 }

+static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
+                               unsigned long end, struct mm_walk *walk)
+
+{
+       struct madvise_free_private *fp = walk->private;
+       struct mmu_gather *tlb = fp->tlb;
+       struct mm_struct *mm = tlb->mm;
+       struct vm_area_struct *vma = fp->vma;
+       spinlock_t *ptl;
+       pte_t *pte, ptent;
+       struct page *page;
+
+       split_huge_page_pmd(vma, addr, pmd);
+       if (pmd_trans_unstable(pmd))
+               return 0;
+
+       pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
+       arch_enter_lazy_mmu_mode();
+       for (; addr != end; pte++, addr += PAGE_SIZE) {
+               ptent = *pte;
+
+               if (!pte_present(ptent))
+                       continue;
+
+               page = vm_normal_page(vma, addr, ptent);
+               if (!page)
+                       continue;
+
+               if (PageSwapCache(page)) {
+                       if (!trylock_page(page))
+                               continue;
+
+                       if (!try_to_free_swap(page)) {
+                               unlock_page(page);
+                               continue;
+                       }
+
+                       ClearPageDirty(page);
+                       unlock_page(page);
+               }
+
+               /*
+                * Some of architecture(ex, PPC) don't update TLB
+                * with set_pte_at and tlb_remove_tlb_entry so for
+                * the portability, remap the pte with old|clean
+                * after pte clearing.
+                */
+               ptent = ptep_get_and_clear_full(mm, addr, pte,
+                                               tlb->fullmm);
+               ptent = pte_mkold(ptent);
+               ptent = pte_mkclean(ptent);
+               set_pte_at(mm, addr, pte, ptent);
+               tlb_remove_tlb_entry(tlb, pte, addr);
+       }
+       arch_leave_lazy_mmu_mode();
+       pte_unmap_unlock(pte - 1, ptl);
+       cond_resched();
+       return 0;
+}
+
+static void madvise_free_page_range(struct mmu_gather *tlb,
+                            struct vm_area_struct *vma,
+                            unsigned long addr, unsigned long end)
+{
+       struct madvise_free_private fp = {
+               .vma = vma,
+               .tlb = tlb,
+       };
+
+       struct mm_walk free_walk = {
+               .pmd_entry = madvise_free_pte_range,
+               .mm = vma->vm_mm,
+               .private = &fp,
+       };
+
+       BUG_ON(addr >= end);
+       tlb_start_vma(tlb, vma);
+       walk_page_range(addr, end, &free_walk);
+       tlb_end_vma(tlb, vma);
+}
+
+static int madvise_free_single_vma(struct vm_area_struct *vma,
+                       unsigned long start_addr, unsigned long end_addr)
+{
+       unsigned long start, end;
+       struct mm_struct *mm = vma->vm_mm;
+       struct mmu_gather tlb;
+
+       if (vma->vm_flags & (VM_LOCKED|VM_HUGETLB|VM_PFNMAP))
+               return -EINVAL;
+
+       /* MADV_FREE works for only anon vma at the moment */
+       if (vma->vm_file)
+               return -EINVAL;
+
+       start = max(vma->vm_start, start_addr);
+       if (start >= vma->vm_end)
+               return -EINVAL;
+       end = min(vma->vm_end, end_addr);
+       if (end <= vma->vm_start)
+               return -EINVAL;
+
+       lru_add_drain();
+       tlb_gather_mmu(&tlb, mm, start, end);
+       update_hiwater_rss(mm);
+
+       mmu_notifier_invalidate_range_start(mm, start, end);
+       madvise_free_page_range(&tlb, vma, start, end);
+       mmu_notifier_invalidate_range_end(mm, start, end);
+       tlb_finish_mmu(&tlb, start, end);
+
+       return 0;
+}
+
+static long madvise_free(struct vm_area_struct *vma,
+                            struct vm_area_struct **prev,
+                            unsigned long start, unsigned long end)
+{
+       *prev = vma;
+       return madvise_free_single_vma(vma, start, end);
+}
+
 /*
  * Application no longer needs these pages.  If the pages are dirty,
  * it's OK to just throw them away.  The app will be more careful about

@@ -381,6 +512,14 @@ madvise_vma(struct vm_area_struct *vma, struct vm_area_struct **prev,
                return madvise_remove(vma, prev, start, end);
        case MADV_WILLNEED:
                return madvise_willneed(vma, prev, start, end);
+       case MADV_FREE:
+               /*
+                * XXX: In this implementation, MADV_FREE works like
+                * MADV_DONTNEED on swapless system or full swap.
+                */
+               if (get_nr_swap_pages() > 0)
+                       return madvise_free(vma, prev, start, end);
+               /* passthrough */
        case MADV_DONTNEED:
                return madvise_dontneed(vma, prev, start, end);
        default:

@@ -400,6 +539,7 @@ madvise_behavior_valid(int behavior)
        case MADV_REMOVE:
        case MADV_WILLNEED:
        case MADV_DONTNEED:
+       case MADV_FREE:
 #ifdef CONFIG_KSM
        case MADV_MERGEABLE:
        case MADV_UNMERGEABLE:

diff --git a/mm/rmap.c b/mm/rmap.c
index 5fbd0fe8f933..93149c82a5a4 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c

@@ -663,6 +663,7 @@ int page_mapped_in_vma(struct page *page, struct vm_area_struct *vma)
 }

 struct page_referenced_arg {
+       int dirtied;
        int mapcount;
        int referenced;
        unsigned long vm_flags;

@@ -677,6 +678,7 @@ static int page_referenced_one(struct page *page, struct vm_area_struct *vma,
        struct mm_struct *mm = vma->vm_mm;
        spinlock_t *ptl;
        int referenced = 0;
+       int dirty = 0;
        struct page_referenced_arg *pra = arg;

        if (unlikely(PageTransHuge(page))) {

@@ -700,6 +702,11 @@ static int page_referenced_one(struct page *page, struct vm_area_struct *vma,
                /* go ahead even if the pmd is pmd_trans_splitting() */
                if (pmdp_clear_flush_young_notify(vma, address, pmd))
                        referenced++;
+
+               /*
+                * In this implmentation, MADV_FREE doesn't support THP free
+                */
+               dirty++;
                spin_unlock(ptl);
        } else {
                pte_t *pte;

@@ -729,6 +736,10 @@ static int page_referenced_one(struct page *page, struct vm_area_struct *vma,
                        if (likely(!(vma->vm_flags & VM_SEQ_READ)))
                                referenced++;
                }
+
+               if (pte_dirty(*pte))
+                       dirty++;
+
                pte_unmap_unlock(pte, ptl);
        }

@@ -737,6 +748,9 @@ static int page_referenced_one(struct page *page, struct vm_area_struct *vma,
                pra->vm_flags |= vma->vm_flags;
        }

+       if (dirty)
+               pra->dirtied++;
+
        pra->mapcount--;
        if (!pra->mapcount)
                return SWAP_SUCCESS; /* To break the loop */

@@ -761,6 +775,7 @@ static bool invalid_page_referenced_vma(struct vm_area_struct *vma, void *arg)
  * @is_locked: caller holds lock on the page
  * @memcg: target memory cgroup
  * @vm_flags: collect encountered vma->vm_flags who actually referenced the page
+ * @is_pte_dirty: ptes which have marked dirty bit - used for lazyfree page
  *
  * Quick test_and_clear_referenced for all mappings to a page,
  * returns the number of ptes which referenced the page.

@@ -768,7 +783,8 @@ static bool invalid_page_referenced_vma(struct vm_area_struct *vma, void *arg)
 int page_referenced(struct page *page,
                    int is_locked,
                    struct mem_cgroup *memcg,
-                   unsigned long *vm_flags)
+                   unsigned long *vm_flags,
+                   int *is_pte_dirty)
 {
        int ret;
        int we_locked = 0;

@@ -783,6 +799,9 @@ int page_referenced(struct page *page,
        };

        *vm_flags = 0;
+       if (is_pte_dirty)
+               *is_pte_dirty = 0;
+
        if (!page_mapped(page))
                return 0;

@@ -810,6 +829,9 @@ int page_referenced(struct page *page,
        if (we_locked)
                unlock_page(page);

+       if (is_pte_dirty)
+               *is_pte_dirty = pra.dirtied;
+
        return pra.referenced;
 }

@@ -1128,6 +1150,7 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
        spinlock_t *ptl;
        int ret = SWAP_AGAIN;
        enum ttu_flags flags = (enum ttu_flags)arg;
+       int dirty = 0;

        pte = page_check_address(page, mm, address, &ptl, 0);
        if (!pte)

@@ -1157,7 +1180,8 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
        pteval = ptep_clear_flush(vma, address, pte);

        /* Move the dirty bit to the physical page now the pte is gone. */
-       if (pte_dirty(pteval))
+       dirty = pte_dirty(pteval);
+       if (dirty)
                set_page_dirty(page);

        /* Update high watermark before we lower rss */

@@ -1186,6 +1210,19 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
                swp_entry_t entry = { .val = page_private(page) };
                pte_t swp_pte;

+               if (flags & TTU_FREE) {
+                       VM_BUG_ON_PAGE(PageSwapCache(page), page);
+                       if (!dirty && !PageDirty(page)) {
+                               /* It's a freeable page by MADV_FREE */
+                               dec_mm_counter(mm, MM_ANONPAGES);
+                               goto discard;
+                       } else {
+                               set_pte_at(mm, address, pte, pteval);
+                               ret = SWAP_FAIL;
+                               goto out_unmap;
+                       }
+               }
+
                if (PageSwapCache(page)) {
                        /*
                         * Store the swap location in the pte.

@@ -1227,6 +1264,7 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
        } else
                dec_mm_counter(mm, MM_FILEPAGES);

+discard:
        page_remove_rmap(page);
        page_cache_release(page);

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 6484195c6859..e3e026c6c7b1 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -713,13 +713,17 @@ enum page_references {
 };

 static enum page_references page_check_references(struct page *page,
-                                                 struct scan_control *sc)
+                                                 struct scan_control *sc,
+                                                 bool *freeable)
 {
        int referenced_ptes, referenced_page;
        unsigned long vm_flags;
+       int pte_dirty;
+
+       VM_BUG_ON_PAGE(!PageLocked(page), page);

        referenced_ptes = page_referenced(page, 1, sc->target_mem_cgroup,
-                                         &vm_flags);
+                                         &vm_flags, &pte_dirty);
        referenced_page = TestClearPageReferenced(page);

        /*

@@ -760,6 +764,10 @@ static enum page_references page_check_references(struct page *page,
                return PAGEREF_KEEP;
        }

+       if (PageAnon(page) && !pte_dirty && !PageSwapCache(page) &&
+                       !PageDirty(page))
+               *freeable = true;
+
        /* Reclaim if clean, defer dirty pages to writeback */
        if (referenced_page && !PageSwapBacked(page))
                return PAGEREF_RECLAIM_CLEAN;

@@ -828,6 +836,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
                int may_enter_fs;
                enum page_references references = PAGEREF_RECLAIM_CLEAN;
                bool dirty, writeback;
+               bool freeable = false;

                cond_resched();

@@ -950,7 +959,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
                }

                if (!force_reclaim)
-                       references = page_check_references(page, sc);
+                       references = page_check_references(page, sc,
+                                                       &freeable);

                switch (references) {
                case PAGEREF_ACTIVATE:

@@ -966,7 +976,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
                 * Anonymous process memory has backing store?
                 * Try to allocate it some swap space here.
                 */
-               if (PageAnon(page) && !PageSwapCache(page)) {
+               if (PageAnon(page) && !PageSwapCache(page) && !freeable) {
                        if (!(sc->gfp_mask & __GFP_IO))
                                goto keep_locked;
                        if (!add_to_swap(page, page_list))

@@ -981,8 +991,9 @@ static unsigned long shrink_page_list(struct list_head *page_list,
                 * The page is mapped into the page tables of one or more
                 * processes. Try to unmap it here.
                 */
-               if (page_mapped(page) && mapping) {
-                       switch (try_to_unmap(page, ttu_flags)) {
+               if (page_mapped(page) && (mapping || freeable)) {
+                       switch (try_to_unmap(page,
+                               freeable ? TTU_FREE : ttu_flags)) {
                        case SWAP_FAIL:
                                goto activate_locked;
                        case SWAP_AGAIN:

@@ -990,7 +1001,20 @@ static unsigned long shrink_page_list(struct list_head *page_list,
                        case SWAP_MLOCK:
                                goto cull_mlocked;
                        case SWAP_SUCCESS:
-                               ; /* try to free the page below */
+                               /* try to free the page below */
+                               if (!freeable)
+                                       break;
+                               /*
+                                * Freeable anon page doesn't have mapping
+                                * due to skipping of swapcache so we free
+                                * page in here rather than __remove_mapping.
+                                */
+                               VM_BUG_ON_PAGE(PageSwapCache(page), page);
+                               if (!page_freeze_refs(page, 1))
+                                       goto keep_locked;
+                               __clear_page_locked(page);
+                               count_vm_event(PGLAZYFREED);
+                               goto free_it;
                        }
                }

@@ -1730,7 +1754,7 @@ static void shrink_active_list(unsigned long nr_to_scan,
                }

                if (page_referenced(page, 0, sc->target_mem_cgroup,
-                                   &vm_flags)) {
+                                   &vm_flags, NULL)) {
                        nr_rotated += hpage_nr_pages(page);
                        /*
                         * Identify referenced, file-backed active pages and

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 87273cddeca0..0b8fc1ac591a 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c

@@ -817,6 +817,7 @@ const char * const vmstat_text[] = {

        "pgfault",
        "pgmajfault",
+       "pglazyfreed",

        TEXTS_FOR_ZONES("pgrefill")
        TEXTS_FOR_ZONES("pgsteal_kswapd")
--

2.0.0

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