diff options
| author | Barry Song <v-songbaohua@oppo.com> | 2024-03-08 22:27:21 +1300 |
|---|---|---|
| committer | Andrew Morton <akpm@linux-foundation.org> | 2024-03-13 12:12:21 -0700 |
| commit | cd197c3a2040100fd8668b33e72b07d4790b39d7 (patch) | |
| tree | db17530befb68566decf3465d23cf9df0f34f1fb /mm | |
| parent | c087a5c324e5577f2d5abee5627c9264c2001ed9 (diff) | |
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.c | 10 |
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. |