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