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Date: Thu, 12 Apr 2018 07:27:18 -0700
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v2 2/2] slab: __GFP_ZERO is incompatible with a
 constructor
Message-ID: <20180412142718.GA20398@bombadil.infradead.org>
References: <20180411060320.14458-1-willy@infradead.org>
 <20180411060320.14458-3-willy@infradead.org>
 <alpine.DEB.2.20.1804110842560.3788@nuc-kabylake>
 <20180411192448.GD22494@bombadil.infradead.org>
 <alpine.DEB.2.20.1804111601090.7458@nuc-kabylake>
 <20180411235652.GA28279@bombadil.infradead.org>
 <alpine.DEB.2.20.1804120907100.11220@nuc-kabylake>
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To: Christopher Lameter <cl@linux.com>
Cc: linux-mm@kvack.org, Matthew Wilcox <mawilcox@microsoft.com>, Pekka Enberg <penberg@kernel.org>, David Rientjes <rientjes@google.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org, Jan Kara <jack@suse.cz>, Jeff Layton <jlayton@redhat.com>, Mel Gorman <mgorman@techsingularity.net>

On Thu, Apr 12, 2018 at 09:10:23AM -0500, Christopher Lameter wrote:
> On Wed, 11 Apr 2018, Matthew Wilcox wrote:
> > I don't see how that works ... can you explain a little more?
>
> c->freelist is NULL and thus ___slab_alloc (slowpath) is called.
> ___slab_alloc populates c->freelist and gets the new object pointer.
> 
> if debugging is on then c->freelist is set to NULL at the end of
> ___slab_alloc because deactivate_slab() is called.
> 
> Thus the next invocation of the fastpath will find that c->freelist is
> NULL and go to the slowpath. ...

_ah_.  I hadn't figured out that c->page was always NULL in the debugging
case too, so ___slab_alloc() always hits the 'new_slab' case.  Thanks!