linux-kernel - Re: [PATCH] slab: perform inc_slabs_node() as part of new

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Message-ID: <53f0cc42-e19a-4b84-8631-390cc5b178f5@suse.cz>
Date: Wed, 22 Oct 2025 19:59:49 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Andrew Morton <akpm@...ux-foundation.org>,
 Christoph Lameter <cl@...two.org>, David Rientjes <rientjes@...gle.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>, Harry Yoo <harry.yoo@...cle.com>,
 Alexei Starovoitov <ast@...nel.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] slab: perform inc_slabs_node() as part of new_slab()

On 10/22/25 19:23, 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 and also make the code more robust by performing
> inc_slabs_node() in new_slab() itself, and removing it from its callers.
> As a side effect, in the theoretical case where the new slab is
> immediately leaked due to debugging consistency check failure, it will
> be accounted (as full) in /proc/slabinfo anyway, which is not wrong.
> 
> The exceptional caller is early_kmem_cache_node_alloc() where the node
> for stats is not yet initialized. We can handle it by using
> allocate_slab() there as the gfp flags are known and fixed and we don't
> need need new_slab()'s fixups.
> 
> Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Pushed to slab/for-next-fixes for testing.

Note I haven't seen the imbalance happening. During the ongoing development
work I caused a very deterministic underflow elsewhere, decided to make the
code more robust by creating this patch, and when going through new_slab()
callers I realized it fixes an existing issue.