From: Barry Song v-songbaohua@oppo.com
mainline inclusion from mainline-v6.9-rc1 commit cd197c3a2040100fd8668b33e72b07d4790b39d7 category: bugfix bugzilla: https://gitee.com/openeuler/kernel/issues/I9S4Z4 CVE: NA
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In a Copy-on-Write (CoW) scenario, the last subpage will reuse the entire large folio, resulting in the waste of (nr_pages - 1) pages. This wasted memory remains allocated until it is either unmapped or memory reclamation occurs.
The following small program can serve as evidence of this behavior
main() { #define SIZE 1024 * 1024 * 1024UL void *p = malloc(SIZE); memset(p, 0x11, SIZE); if (fork() == 0) _exit(0); memset(p, 0x12, SIZE); printf("done\n"); while(1); }
For example, using a 1024KiB mTHP by: echo always > /sys/kernel/mm/transparent_hugepage/hugepages-1024kB/enabled
(1) w/o the patch, it takes 2GiB,
Before running the test program, / # free -m total used free shared buff/cache available Mem: 5754 84 5692 0 17 5669 Swap: 0 0 0
/ # /a.out & / # done
After running the test program, / # free -m total used free shared buff/cache available Mem: 5754 2149 3627 0 19 3605 Swap: 0 0 0
(2) w/ the patch, it takes 1GiB only,
Before running the test program, / # free -m total used free shared buff/cache available Mem: 5754 89 5687 0 17 5664 Swap: 0 0 0
/ # /a.out & / # done
After running the test program, / # free -m total used free shared buff/cache available Mem: 5754 1122 4655 0 17 4632 Swap: 0 0 0
This patch migrates the last subpage to a small folio and immediately returns the large folio to the system. It benefits both memory availability and anti-fragmentation.
Link: https://lkml.kernel.org/r/20240308092721.144735-1-21cnbao@gmail.com Signed-off-by: Barry Song v-songbaohua@oppo.com Acked-by: David Hildenbrand david@redhat.com Cc: Ryan Roberts ryan.roberts@arm.com Cc: Lance Yang ioworker0@gmail.com Signed-off-by: Andrew Morton akpm@linux-foundation.org (cherry picked from commit cd197c3a2040100fd8668b33e72b07d4790b39d7) Signed-off-by: Kefeng Wang wangkefeng.wang@huawei.com --- mm/memory.c | 10 ++++++++++ 1 file changed, 10 insertions(+)
diff --git a/mm/memory.c b/mm/memory.c index 48d08cdc1a1c..9d264c303972 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3532,6 +3532,16 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf, struct folio *folio) static bool wp_can_reuse_anon_folio(struct folio *folio, struct vm_area_struct *vma) { + /* + * We could currently only reuse a subpage of a large folio if no + * other subpages of the large folios are still mapped. However, + * let's just consistently not reuse subpages even if we could + * reuse in that scenario, and give back a large folio a bit + * sooner. + */ + if (folio_test_large(folio)) + return false; + /* * We have to verify under folio lock: these early checks are * just an optimization to avoid locking the folio and freeing