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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Thu, Apr 03, 2025 at 09:43:39AM +0200, Michal Hocko wrote:
> 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>

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>