Messages in this thread Patch in this message |  | | From | Barry Song <> | | Subject | [PATCH] mm: prohibit the last subpage from reusing the entire large folio | | Date | Fri, 8 Mar 2024 21:56:53 +1300 |
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From: Barry Song <v-songbaohua@oppo.com>
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.
Cc: David Hildenbrand <david@redhat.com> Cc: Ryan Roberts <ryan.roberts@arm.com> Cc: Lance Yang <ioworker0@gmail.com> Signed-off-by: Barry Song <v-songbaohua@oppo.com> --- mm/memory.c | 8 ++++++++ 1 file changed, 8 insertions(+)
diff --git a/mm/memory.c b/mm/memory.c index e17669d4f72f..0200bfc15f94 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3523,6 +3523,14 @@ static bool wp_can_reuse_anon_folio(struct folio *folio, folio_unlock(folio); return false; } + /* + * If the last subpage reuses the entire large folio, it would + * result in a waste of (nr_pages - 1) pages + */ + if (folio_ref_count(folio) == 1 && folio_test_large(folio)) { + folio_unlock(folio); + return false; + } /* * Ok, we've got the only folio reference from our mapping * and the folio is locked, it's dark out, and we're wearing -- 2.34.1
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