Re: [PATCH v3] mm: make minimum slab alignment a runtime property

[David Rientjes <rientjes@google.com>]

On Fri, 22 Apr 2022, Peter Collingbourne wrote:

> When CONFIG_KASAN_HW_TAGS is enabled we currently increase the minimum
> slab alignment to 16. This happens even if MTE is not supported in
> hardware or disabled via kasan=off, which creates an unnecessary
> memory overhead in those cases. Eliminate this overhead by making
> the minimum slab alignment a runtime property and only aligning to
> 16 if KASAN is enabled at runtime.
> 
> On a DragonBoard 845c (non-MTE hardware) with a kernel built with
> CONFIG_KASAN_HW_TAGS, waiting for quiescence after a full Android
> boot I see the following Slab measurements in /proc/meminfo (median
> of 3 reboots):
> 
> Before: 169020 kB
> After:  167304 kB
> 
> Link: https://linux-review.googlesource.com/id/I752e725179b43b144153f4b6f584ceb646473ead
> Signed-off-by: Peter Collingbourne <pcc@google.com>
> Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>

Acked-by: David Rientjes <rientjes@google.com>

The command line options are described by 
Documentation/dev-tools/kasan.rst but it doesn't look like a update is 
necessary.  I think the assumption today is that if we're using kasan=off 
then we aren't doing the alignment.

I do wonder why kasan=off is not at least mentioned in 
Documentation/admin-guide/kernel-parameters.txt and perhaps for all other 
kasan options point the reader to Documentation/dev-tools/kasan.rst.