linux-kernel - Re: [PATCH 1/2] slab: __GFP_ZERO is incompatible with a constructor

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Message-ID: <alpine.DEB.2.20.1804101512350.30225@nuc-kabylake>
Date:   Tue, 10 Apr 2018 15:21:45 -0500 (CDT)
From:   Christopher Lameter <cl@...ux.com>
To:     Matthew Wilcox <willy@...radead.org>
cc:     Eric Dumazet <eric.dumazet@...il.com>, linux-mm@...ck.org,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/2] slab: __GFP_ZERO is incompatible with a
 constructor

On Tue, 10 Apr 2018, Matthew Wilcox wrote:

> > 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.

But the object needs to have the proper contents after it was released and
re-allocated. Some objects may rely on contents (like list heads)
surviving the realloc process because access must always be possible.

validate_slab() checks on proper metadata content in a slab
although it does not access the payload. So that may work you separate
the payload init from the metadata init.