linux-kernel - Re: [PATCH v2 2/2] mm, sl[aou]b: guarantee natural alignment for kmalloc(power-of-two)

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Message-ID: <0100016cf8c3033d-bbcc9ba3-2d59-4654-a7c2-8ba094f8a7de-000000@email.amazonses.com>
Date:   Tue, 3 Sep 2019 20:13:45 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Matthew Wilcox <willy@...radead.org>
cc:     Michal Hocko <mhocko@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Ming Lei <ming.lei@...hat.com>,
        Dave Chinner <david@...morbit.com>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@....de>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        linux-btrfs@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm, sl[aou]b: guarantee natural alignment for
 kmalloc(power-of-two)

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.

> > 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?