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Date: Tue, 10 Apr 2018 10:50:12 -0700
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH 1/2] slab: __GFP_ZERO is incompatible with a constructor
Message-ID: <20180410175011.GE3614@bombadil.infradead.org>
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To: Christopher Lameter <cl@linux.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>, 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>, stable@vger.kernel.org

On Tue, Apr 10, 2018 at 12:45:56PM -0500, Christopher Lameter wrote:
> On Tue, 10 Apr 2018, Matthew Wilcox wrote:
> 
> > > How do you envision dealing with the SLAB_TYPESAFE_BY_RCU slab caches?
> > > Those must have a defined state of the objects at all times and a constructor is
> > > required for that. And their use of RCU is required for numerous lockless
> > > lookup algorithms in the kernhel.
> >
> > Not at all times.  Only once they've been used.  Re-constructing them
> > once they've been used might break the rcu typesafety, I suppose ...
> > would need to examine the callers.
> 
> Objects can be freed and reused and still be accessed from code that
> thinks the object is the old and not the new object....

Yes, I know, that's the point of RCU typesafety.  My point is that an
object *which has never been used* can't be accessed.  So you don't *need*
a constructor.