linux-kernel - Re: [PATCH v2 1/2] mm, slab: move memcg charging to post-alloc hook

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Date: Sat, 13 Apr 2024 21:55:49 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Jeff Layton <jlayton@...nel.org>, 
	Chuck Lever <chuck.lever@...cle.com>, Kees Cook <kees@...nel.org>, Christoph Lameter <cl@...ux.com>, 
	Pekka Enberg <penberg@...nel.org>, David Rientjes <rientjes@...gle.com>, 
	Joonsoo Kim <iamjoonsoo.kim@....com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Hyeonggon Yoo <42.hyeyoo@...il.com>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Muchun Song <muchun.song@...ux.dev>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	Chengming Zhou <chengming.zhou@...ux.dev>
Subject: Re: [PATCH v2 1/2] mm, slab: move memcg charging to post-alloc hook

On Mon, Mar 25, 2024 at 09:20:32AM +0100, Vlastimil Babka wrote:
> The MEMCG_KMEM integration with slab currently relies on two hooks
> during allocation. memcg_slab_pre_alloc_hook() determines the objcg and
> charges it, and memcg_slab_post_alloc_hook() assigns the objcg pointer
> to the allocated object(s).
> 
> As Linus pointed out, this is unnecessarily complex. Failing to charge
> due to memcg limits should be rare, so we can optimistically allocate
> the object(s) and do the charging together with assigning the objcg
> pointer in a single post_alloc hook. In the rare case the charging
> fails, we can free the object(s) back.
> 
> This simplifies the code (no need to pass around the objcg pointer) and
> potentially allows to separate charging from allocation in cases where
> it's common that the allocation would be immediately freed, and the
> memcg handling overhead could be saved.
> 
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Link: https://lore.kernel.org/all/CAHk-=whYOOdM7jWy5jdrAm8LxcgCMFyk2bt8fYYvZzM4U-zAQA@mail.gmail.com/
> Reviewed-by: Roman Gushchin <roman.gushchin@...ux.dev>
> Reviewed-by: Chengming Zhou <chengming.zhou@...ux.dev>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

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