linux-kernel - Re: [PATCH v3 2/2] slab: __GFP_ZERO is incompatible with a constructor

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Date:   Fri, 3 Aug 2018 15:33:57 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Christopher Lameter <cl@...ux.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>,
        linux-sh@...r.kernel.org
Subject: Re: [PATCH v3 2/2] slab: __GFP_ZERO is incompatible with a
 constructor

On Fri, Aug 03, 2018 at 02:22:57PM -0700, Guenter Roeck wrote:
> Hi,
> 
> On Thu, Apr 12, 2018 at 12:13:22PM -0700, Matthew Wilcox wrote:
> > From: Matthew Wilcox <mawilcox@...rosoft.com>
> > 
> > __GFP_ZERO requests that the object be initialised to all-zeroes,
> > while the purpose of a constructor is to initialise an object to a
> > particular pattern.  We cannot do both.  Add a warning to catch any
> > users who mistakenly pass a __GFP_ZERO flag when allocating a slab with
> > a constructor.
> > 
> > Fixes: d07dbea46405 ("Slab allocators: support __GFP_ZERO in all allocators")
> > Signed-off-by: Matthew Wilcox <mawilcox@...rosoft.com>
> > Acked-by: Johannes Weiner <hannes@...xchg.org>
> > Acked-by: Vlastimil Babka <vbabka@...e.cz>
> > Acked-by: Michal Hocko <mhocko@...e.com>
> 
> Seen with v4.18-rc7-139-gef46808 and v4.18-rc7-178-g0b5b1f9a78b5 when
> booting sh4 images in qemu:

Thanks!  It's under discussion here:

https://marc.info/?t=153301426900002&r=1&w=2

also reported here with a bogus backtrace:

https://marc.info/?l=linux-sh&m=153305755505935&w=2

Short version: It's a bug that's been present since 2009 and nobody
noticed until now.  And nobody's quite sure what the effect of this
bug is.