linux-kernel - Re: [PATCH] mm/slab.c: use 'ac' from the caller

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Date:   Wed, 21 Apr 2021 09:20:41 +0200 (CEST)
From:   Christoph Lameter <cl@...two.de>
To:     Matthew Wilcox <willy@...radead.org>
cc:     Yejune Deng <yejune.deng@...il.com>, penberg@...nel.org,
        rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, vbabka@...e.cz, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/slab.c: use 'ac' from the caller

On Wed, 21 Apr 2021, Matthew Wilcox wrote:

> On Wed, Apr 21, 2021 at 10:25:17AM +0800, Yejune Deng wrote:
> > @@ -3045,12 +3044,7 @@ static inline void *____cache_alloc(struct kmem_cache *cachep, gfp_t flags)
> >  	}
> >
> >  	STATS_INC_ALLOCMISS(cachep);
> > -	objp = cache_alloc_refill(cachep, flags);
> > -	/*
> > -	 * the 'ac' may be updated by cache_alloc_refill(),
> > -	 * and kmemleak_erase() requires its correct value.
> > -	 */
> > -	ac = cpu_cache_get(cachep);
> > +	objp = cache_alloc_refill(cachep, ac, flags);
>
> I think passing 'ac' in is fine (probably?  I don't know this code
> deeply), but deleting this call to 'ac' is clearly wrong.  The comment
> even tells you that!  I just verified the code, and the comment is
> correct.


Yep the delete of the ac assignment is wrong.

But even without that issue: There is no point to passing ac to
cache_alloc_refill since cpu_cache_get is rather trivial and does not
even require memory access since "cachep" is usually in some register.