linux-kernel - Re: [PATCH v2] slab: fix slab accounting imbalance due to defer_deactivate

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Message-ID: <aPomAwgmrHSqOAb5@hyeyoo>
Date: Thu, 23 Oct 2025 21:56:35 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...two.org>,
        David Rientjes <rientjes@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Alexei Starovoitov <ast@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] slab: fix slab accounting imbalance due to
 defer_deactivate_slab()

On Thu, Oct 23, 2025 at 02:01:07PM +0200, Vlastimil Babka wrote:
> Since commit af92793e52c3 ("slab: Introduce kmalloc_nolock() and
> kfree_nolock().") there's a possibility in alloc_single_from_new_slab()
> that we discard the newly allocated slab if we can't spin and we fail to
> trylock. As a result we don't perform inc_slabs_node() later in the
> function. Instead we perform a deferred deactivate_slab() which can
> either put the unacounted slab on partial list, or discard it
> immediately while performing dec_slabs_node(). Either way will cause an
> accounting imbalance.
> 
> Fix this by not marking the slab as frozen, and using free_slab()
> instead of deactivate_slab() for non-frozen slabs in
> free_deferred_objects(). For CONFIG_SLUB_TINY, that's the only possible
> case. By not using discard_slab() we avoid dec_slabs_node().
> 
> Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
> Changes in v2:
> - Fix the problem differently. Harry pointed out that we can't move
>   inc_slabs_node() outside of list_lock protected regions as that would
>   reintroduce issues fixed by commit c7323a5ad078
> - Link to v1: https://patch.msgid.link/20251022-fix-slab-accounting-v1-1-27870ec363ce@suse.cz
> ---

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@...cle.com>

-- 
Cheers,
Harry / Hyeonggon