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

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Message-ID: <b696e3c2-3d96-4729-9e07-87bb644f145b@gmail.com>
Date: Tue, 20 May 2025 15:13:44 +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 14:46, Usama Arif wrote:
> 
> 
> 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?

Also a majority of them are less than 1 page. kvmalloc of less than 1 page
falls back to kmalloc. So vmalloc will only be on those greater than 1 page
size, which are in the minority (for e.g. zs_handle, request_sock_subflow_v6,
request_sock_subflow_v4...).