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

[Guenter Roeck]

On 08/04/2018 02:28 AM, Geert Uytterhoeven wrote:
> On Sat, Aug 4, 2018 at 12:34 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > On Fri, Aug 03, 2018 at 02:22:57PM -0700, Guenter Roeck wrote:
> > > On Thu, Apr 12, 2018 at 12:13:22PM -0700, Matthew Wilcox wrote:
> > > > From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
> > > > __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@xxxxxxxxxxxxx>
> > > > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > > Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> > > > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> > >
> > > 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
>
> and https://www.spinics.net/lists/linux-sh/msg53298.html
>
> > 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.

Though now it is making a lot of noise :-).

I just found two more 0-day bugs, so maybe improved testing and log messages
such as the one encountered here do help a bit.

Guenter