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Date:   Tue, 3 Sep 2019 13:53:12 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Christopher Lameter <cl@linux.com>
Cc:     Michal Hocko <mhocko@kernel.org>, Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Ming Lei <ming.lei@redhat.com>,
        Dave Chinner <david@fromorbit.com>,
        "Darrick J . Wong" <darrick.wong@oracle.com>,
        Christoph Hellwig <hch@lst.de>, linux-xfs@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        linux-btrfs@vger.kernel.org
Subject: Re: [PATCH v2 2/2] mm, sl[aou]b: guarantee natural alignment for
 kmalloc(power-of-two)
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On Tue, Sep 03, 2019 at 08:13:45PM +0000, Christopher Lameter wrote:
> On Sat, 31 Aug 2019, Matthew Wilcox wrote:
> 
> > > The current behavior without special alignment for these caches has been
> > > in the wild for over a decade. And this is now coming up?
> >
> > In the wild ... and rarely enabled.  When it is enabled, it may or may
> > not be noticed as data corruption, or tripping other debugging asserts.
> > Users then turn off the rare debugging option.
> 
> Its enabled in all full debug session as far as I know. Fedora for
> example has been running this for ages to find breakage in device drivers
> etc etc.

Are you telling me nobody uses the ramdisk driver on fedora?  Because
that's one of the affected drivers.

> > > If there is an exceptional alignment requirement then that needs to be
> > > communicated to the allocator. A special flag or create a special
> > > kmem_cache or something.
> >
> > The only way I'd agree to that is if we deliberately misalign every
> > allocation that doesn't have this special flag set.  Because right now,
> > breakage happens everywhere when these debug options are enabled, and
> > the very people who need to be helped are being hurt by the debugging.
> 
> That is customarily occurring for testing by adding "slub_debug" to the
> kernel commandline (or adding debug kernel options) and since my
> information is that this is done frequently (and has been for over a
> decade now) I am having a hard time believing the stories of great
> breakage here. These drivers were not tested with debugging on before?
> Never ran with a debug kernel?

Whatever is being done is clearly not enough to trigger the bug.  So how
about it?  Create an option to slab/slub to always return misaligned
memory.