linux-kernel - Re: [PATCH v2 01/15] slab: correct pfmemalloc check

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Message-ID: <20131017052730.GA26617@lge.com>
Date:	Thu, 17 Oct 2013 14:27:31 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Pekka Enberg <penberg@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v2 01/15] slab: correct pfmemalloc check

On Wed, Oct 16, 2013 at 03:27:54PM +0000, Christoph Lameter wrote:
> On Wed, 16 Oct 2013, Joonsoo Kim wrote:
> 
> > --- a/mm/slab.c
> > +++ b/mm/slab.c
> > @@ -930,7 +930,8 @@ static void *__ac_put_obj(struct kmem_cache *cachep, struct array_cache *ac,
> >  {
> >  	if (unlikely(pfmemalloc_active)) {
> >  		/* Some pfmemalloc slabs exist, check if this is one */
> > -		struct page *page = virt_to_head_page(objp);
> > +		struct slab *slabp = virt_to_slab(objp);
> > +		struct page *page = virt_to_head_page(slabp->s_mem);
> >  		if (PageSlabPfmemalloc(page))
> 
> I hope the compiler optimizes this code correctly because virt_to_slab
> already does one virt_to_head_page()?

It should not.
objp could be in a different page with slabp->s_mem's,
so virt_to_head_page() should be called twice.

Anyway, after implementing struct slab overloading, one call site is
removed by [14/15] in this patchset, so there is no issue.

Thanks.

> 
> Otherwise this looks fine.
> 
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