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authorBarry Song <v-songbaohua@oppo.com>2024-03-08 12:27:21 +0300
committerAndrew Morton <akpm@linux-foundation.org>2024-03-13 22:12:21 +0300
commitcd197c3a2040100fd8668b33e72b07d4790b39d7 (patch)
treedb17530befb68566decf3465d23cf9df0f34f1fb /mm
parentc087a5c324e5577f2d5abee5627c9264c2001ed9 (diff)
downloadlinux-cd197c3a2040100fd8668b33e72b07d4790b39d7.tar.xz
mm: prohibit the last subpage from reusing the entire large folio
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>
Diffstat (limited to 'mm')
-rw-r--r--mm/memory.c10
1 files changed, 10 insertions, 0 deletions
diff --git a/mm/memory.c b/mm/memory.c
index e17669d4f72f..f2bc6dd15eb8 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3499,6 +3499,16 @@ 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
* the swapcache if there is little hope that we can reuse.