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

Christopher Lameter
2019-09-03T20:13:50Z

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?