linux-kernel - Re: [PATCH] mm/slab: add new flag SLAB_NO

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Date:   Wed, 24 May 2023 21:56:02 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     David Sterba <dsterba@...e.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Roman Gushchin <roman.gushchin@...ux.dev>
Subject: Re: [PATCH] mm/slab: add new flag SLAB_NO_MERGE to avoid merging per
 slab

On Wed, May 24, 2023 at 12:17:48PM +0200, David Sterba wrote:
> Add a flag that allows to disable merging per slab. This can be used for
> more fine grained control over the caches or for debugging builds where
> separate slabs can verify that no objects leak.
> The slab_nomerge boot option is too coarse and would need to be enabled
> on all testing hosts. 

Hello David,

There is no users nor interface to set this flag, I guess you're going
to use it by modifying source code, when debugging?

Does introducing new slub_debug option (i.e. slub_debug=N,pid_namespace)
work for your use case? (there are some boot-time slub_debug options described in
Documentation/mm/slub.rst)

> There are some other ways how to disable merging,
> e.g. a slab constructor but this disables poisoning besides that it adds
> additional overhead. Other flags are internal and may have other
> semantics.
>
> A concrete example what motivates the flag. During 'btrfs balance' slab
> top reported huge increase in caches like
> 
>   1330095 1330095 100%    0.10K  34105       39    136420K Acpi-ParseExt
>   1734684 1734684 100%    0.14K  61953       28    247812K pid_namespace
>   8244036 6873075  83%    0.11K 229001       36    916004K khugepaged_mm_slot
> 
> which was confusing and that it's because of slab merging was not the
> first idea.  After rebooting with slab_nomerge all the caches were from
> btrfs_ namespace as expected.
>
> Signed-off-by: David Sterba <dsterba@...e.com>
> ---
>  include/linux/slab.h | 3 +++
>  mm/slab_common.c     | 2 +-
>  2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h

Thanks,

-- 
Hyeonggon Yoo

Doing kernel stuff as a hobby
Undergraduate | Chungnam National University
Dept. Computer Science & Engineering