linux-kernel - Re: [PATCH v2 2/7] mm, slab: unlink slabinfo, sysfs and debugfs immediately

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Message-ID: <CAG48ez1nuA5bwDUWpwOoMdVqdjhgSKMc+mtFwuH6pbbz51CA_Q@mail.gmail.com>
Date: Wed, 7 Aug 2024 21:11:45 +0200
From: Jann Horn <jannh@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, Joel Fernandes <joel@...lfernandes.org>, 
	Josh Triplett <josh@...htriplett.org>, Boqun Feng <boqun.feng@...il.com>, 
	Christoph Lameter <cl@...ux.com>, David Rientjes <rientjes@...gle.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Lai Jiangshan <jiangshanlai@...il.com>, 
	Zqiang <qiang.zhang1211@...il.com>, Julia Lawall <Julia.Lawall@...ia.fr>, 
	Jakub Kicinski <kuba@...nel.org>, "Jason A. Donenfeld" <Jason@...c4.com>, 
	"Uladzislau Rezki (Sony)" <urezki@...il.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, rcu@...r.kernel.org, 
	Alexander Potapenko <glider@...gle.com>, Marco Elver <elver@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	kasan-dev@...glegroups.com, Mateusz Guzik <mjguzik@...il.com>
Subject: Re: [PATCH v2 2/7] mm, slab: unlink slabinfo, sysfs and debugfs immediately

On Wed, Aug 7, 2024 at 12:31 PM Vlastimil Babka <vbabka@...e.cz> wrote:
> kmem_cache_destroy() includes removing the associated sysfs and debugfs
> directories, and the cache from the list of caches that appears in
> /proc/slabinfo. Currently this might not happen immediately when:
>
> - the cache is SLAB_TYPESAFE_BY_RCU and the cleanup is delayed,
>   including the directores removal
> - __kmem_cache_shutdown() fails due to outstanding objects - the
>   directories remain indefinitely
>
> When a cache is recreated with the same name, such as due to module
> unload followed by a load, the directories will fail to be recreated for
> the new instance of the cache due to the old directories being present.
> The cache will also appear twice in /proc/slabinfo.
>
> While we want to convert the SLAB_TYPESAFE_BY_RCU cleanup to be
> synchronous again, the second point remains. So let's fix this first and
> have the directories and slabinfo removed immediately in
> kmem_cache_destroy() and regardless of __kmem_cache_shutdown() success.
>
> This should not make debugging harder if __kmem_cache_shutdown() fails,
> because a detailed report of outstanding objects is printed into dmesg
> already due to the failure.

Reading this sentence made be curious what __kmem_cache_shutdown()
actually does - and I think technically, it prints a report of only
the outstanding objects *on the first NUMA node with outstanding
objects*? __kmem_cache_shutdown() bails immediately after seeing one
node with outstanding objects.

That's not really relevant to this series though, just a random observation.

> Also simplify kmem_cache_release() sysfs handling by using
> __is_defined(SLAB_SUPPORTS_SYSFS).
>
> Note the resulting code in kmem_cache_destroy() is a bit ugly but will
> be further simplified - this is in order to make small bisectable steps.
>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Reviewed-by: Jann Horn <jannh@...gle.com>