linux-kernel - Re: [PATCH 1/2] mm: slub: allocate slab object extensions non-contiguously

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Message-ID: <6d015d91-e74c-48b3-8bc3-480980a74f9b@gmail.com>
Date: Tue, 20 May 2025 14:46:14 +0100
From: Usama Arif <usamaarif642@...il.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, surenb@...gle.com,
 hannes@...xchg.org, shakeel.butt@...ux.dev, vlad.wing@...il.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH 1/2] mm: slub: allocate slab object extensions
 non-contiguously



On 20/05/2025 14:44, Kent Overstreet wrote:
> On Tue, May 20, 2025 at 01:25:46PM +0100, Usama Arif wrote:
>> When memory allocation profiling is running on memory bound services,
>> allocations greater than order 0 for slab object extensions can fail,
>> for e.g. zs_handle zswap slab which will be 512 objsperslab x 16 bytes
>> per slabobj_ext (order 1 allocation). Use kvcalloc to improve chances
>> of the allocation being successful.
>>
>> Signed-off-by: Usama Arif <usamaarif642@...il.com>
>> Reported-by: Vlad Poenaru <vlad.wing@...il.com>
>> Closes: https://lore.kernel.org/all/17fab2d6-5a74-4573-bcc3-b75951508f0a@gmail.com/
>> ---
>>  mm/slub.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index dc9e729e1d26..bf43c403ead2 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -1989,7 +1989,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>>  	gfp &= ~OBJCGS_CLEAR_MASK;
>>  	/* Prevent recursive extension vector allocation */
>>  	gfp |= __GFP_NO_OBJ_EXT;
>> -	vec = kcalloc_node(objects, sizeof(struct slabobj_ext), gfp,
>> +	vec = kvcalloc_node(objects, sizeof(struct slabobj_ext), gfp,
>>  			   slab_nid(slab));
> 
> And what's the latency going to be on a vmalloc() allocation when we're
> low on memory?

Would it not be better to get the allocation slighly slower than to not get
it at all?