[PATCH] mm: kvmalloc: make kmalloc fast path real fast path

Michal Hocko
2025-04-03T07:43:46Z

There are users like xfs which need larger allocations with NOFAIL
sementic. They are not using kvmalloc currently because the current
implementation tries too hard to allocate through the kmalloc path
which causes a lot of direct reclaim and compaction and that hurts
performance a lot (see 8dc9384b7d75 ("xfs: reduce kvmalloc overhead for
CIL shadow buffers") for more details).

kvmalloc does support __GFP_RETRY_MAYFAIL semantic to express that
kmalloc (physically contiguous) allocation is preferred and we should go
more aggressive to make it happen. There is currently no way to express
that kmalloc should be very lightweight and as it has been argued [1]
this mode should be default to support kvmalloc(NOFAIL) with a
lightweight kmalloc path which is currently impossible to express as
__GFP_NOFAIL cannot be combined by any other reclaim modifiers.

This patch makes all kmalloc allocations GFP_NOWAIT unless
__GFP_RETRY_MAYFAIL is provided to kvmalloc. This allows to support both
fail fast and retry hard on physically contiguous memory with vmalloc
fallback.

There is a potential downside that relatively small allocations (smaller
than PAGE_ALLOC_COSTLY_ORDER) could fallback to vmalloc too easily and
cause page block fragmentation. We cannot really rule that out but it
seems that xlog_cil_kvmalloc use doesn't indicate this to be happening.

[1] https://lore.kernel.org/all/Z-3i1wATGh6vI8x8@dread.disaster.area/T/#u
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
mm/slub.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index b46f87662e71..2da40c2f6478 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4972,14 +4972,16 @@ static gfp_t kmalloc_gfp_adjust(gfp_t flags, size_t size)
* We want to attempt a large physically contiguous block first because
* it is less likely to fragment multiple larger blocks and therefore
* contribute to a long term fragmentation less than vmalloc fallback.
- * However make sure that larger requests are not too disruptive - no
- * OOM killer and no allocation failure warnings as we have a fallback.
+ * However make sure that larger requests are not too disruptive - i.e.
+ * do not direct reclaim unless physically continuous memory is preferred
+ * (__GFP_RETRY_MAYFAIL mode). We still kick in kswapd/kcompactd to start
+ * working in the background but the allocation itself.
*/
if (size > PAGE_SIZE) {
flags |= __GFP_NOWARN;

if (!(flags & __GFP_RETRY_MAYFAIL))
- flags |= __GFP_NORETRY;
+ flags &= ~__GFP_DIRECT_RECLAIM;

/* nofail semantic is implemented by the vmalloc fallback */
flags &= ~__GFP_NOFAIL;
--
2.49.0