From: Barry Song <v-songbaohua@xxxxxxxx>
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@xxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Lance Yang <ioworker0@xxxxxxxxx>
Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
---
-v2:
* return at the 1st beginning for a large folio according to David's comment,
thanks!
-v1:
https://lore.kernel.org/linux-mm/20240308085653.124180-1-21cnbao@xxxxxxxxx/
mm/memory.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/mm/memory.c b/mm/memory.c
index e17669d4f72f..f2bc6dd15eb8 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3498,6 +3498,16 @@ 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
--
2.34.1
