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

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Message-ID: <CAADnVQJXDPFaY6HyB+RTnGAVk9xwv_TZkVxu9MO7EPRktRLndw@mail.gmail.com>
Date: Wed, 22 Oct 2025 12:20:44 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.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>, 
	Harry Yoo <harry.yoo@...cle.com>, Alexei Starovoitov <ast@...nel.org>, linux-mm <linux-mm@...ck.org>, 
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] slab: perform inc_slabs_node() as part of new_slab()

On Wed, Oct 22, 2025 at 11:00 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> 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.

lgtm
Acked-by: Alexei Starovoitov <ast@...nel.org>