From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A4EF425393E for ; Tue, 25 Aug 2026 13:56:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787666173; cv=none; b=V7axlTqTV5UwQZMGuBJahPJv8eYy1OZXbXv7JKH4GS4m8wNTh5qrSEee5DCaqUZt4wCFLc0XJ0zsPSlEZZA6QR5m1SJxLHLRQAhztn5B7Jxae63sGtDj4qWXeLai8T/WwdLJFsJq8SBryduLOclyeu/apiSiZWhLeSdzBL7eiXE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787666173; c=relaxed/simple; bh=Q6r9/xeo6AMgRWddrJ39y/8RYfook89rU22M8hTuofk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=jHJmhXyot+MbYQnmjD1+keoFhSSyA2lZkaf+iYxj2OyyKOkDDoWLp9ZLz/2lYHUrv9IQ+tvbRY1sbo/f2UFQGW53v+nC/raC2gKo/a/qbJACfphWfHWYXO08kDl7vgQmpGWvxsr7WHX+//c23GNYF2AbC9b0HFMOSIqhLC6bvjk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ooeyY7C7; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ooeyY7C7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BF0A71F00A3A; Tue, 25 Aug 2026 13:56:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787666172; bh=FMK9RB8BHr7FFWJqHW9duwXzfQjY/GK6Lj1xCAYHp5Q=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=ooeyY7C7Wy+YNpKjA2xzRnySKqHQGeyZlbDkB4S4jvZJBVaFN2aPfF6a73GXK8j4A hbBC7iaiSbQoXcTQFTcIflyIY5Au6eOqVeMsUuCPe1DE0dSY6EsIMi3f7pQwFDKegI +Icsgu39D8nqhFklbzqRoibaHY7ynIvNy75ys5oijRslY6Y9cdUHuEtBzeh9n8dLCj S2fyYw/sULPat7bu4OQ8fuWvdFBlg3lcArY2ceJVkn9J+goEWcdobH4zjS7HO63K8C WS73BTT8F64SEfZ0Kh8lfhNiUkjGGDi1fWTLbbaJg9T37w1Gd+KGRtc4snyWAWnwhC CJc80O6LyAeKw== Message-ID: <8969b1c2-360b-4fd0-ab70-eb4a68365f50@kernel.org> Date: Tue, 25 Aug 2026 15:56:05 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/shrinker: do not hold RCU lock in shrinker_debugfs_count_show() To: Nick Huang Cc: Shakeel Butt , Andrew Morton , Dave Chinner , Roman Gushchin , Muchun Song , Qi Zheng , Meta kernel team , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Zenghui Yu , Nhat Pham References: <20260610232048.62930-1-shakeel.butt@linux.dev> <041af2b5-9136-4027-b35a-72e5044c63ab@kernel.org> From: Krzysztof Kozlowski Content-Language: en-US Autocrypt: addr=krzk@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 25/08/2026 15:54, Nick Huang wrote: > On Tue, Aug 25, 2026 at 08:12:09AM +0200, Krzysztof Kozlowski wrote: >> On 24/08/2026 15:42, Nick Huang wrote: >>> On Wed, Jun 10, 2026 at 04:20:48PM -0700, Shakeel Butt wrote: >>>> Reading the debugfs "count" file of a memcg-aware shrinker can sleep >>>> inside an RCU read-side critical section: >>>> >>>> BUG: sleeping function called from invalid context at kernel/cgroup/rstat.c:421 >>>> RCU nest depth: 1, expected: 0 >>>> css_rstat_flush >>>> mem_cgroup_flush_stats >>>> zswap_shrinker_count >>>> shrinker_debugfs_count_show >>>> >>>> shrinker_debugfs_count_show() invokes the ->count_objects() callback >>>> under rcu_read_lock(). The zswap callback flushes memcg stats via >>>> css_rstat_flush(), which may sleep, so it must not run under RCU. >>>> >>>> The RCU lock is not needed here. mem_cgroup_iter() takes RCU internally >>>> and returns a memcg holding a css reference (dropped on the next >>>> iteration or by mem_cgroup_iter_break()), so the memcg stays alive >>>> without it. The shrinker is kept alive by the open debugfs file: >>>> shrinker_free() removes the debugfs entries via >>>> debugfs_remove_recursive(), which waits for in-flight readers to drain, >>>> before call_rcu(..., shrinker_free_rcu_cb). The sibling "scan" handler >>>> already invokes the sleeping ->scan_objects() callback with no RCU >>>> section. >>>> >>>> Drop the rcu_read_lock()/rcu_read_unlock(). >>>> >>>> Fixes: 5035ebc644ae ("mm: shrinkers: introduce debugfs interface for memory shrinkers") >>>> Reported-by: Zenghui Yu >>>> Closes: https://lore.kernel.org/all/c052a064-cddb-494f-a0d8-f8a10b4b1c4d@linux.dev/ >>>> Suggested-by: Nhat Pham >>>> Signed-off-by: Shakeel Butt >>>> --- >>>> mm/shrinker_debug.c | 4 ---- >>>> 1 file changed, 4 deletions(-) >>>> >>>> diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c >>>> index affa64437302..cda4e86428c8 100644 >>>> --- a/mm/shrinker_debug.c >>>> +++ b/mm/shrinker_debug.c >>>> @@ -57,8 +57,6 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v) >>>> if (!count_per_node) >>>> return -ENOMEM; >>>> >>>> - rcu_read_lock(); >>>> - >>>> memcg_aware = shrinker->flags & SHRINKER_MEMCG_AWARE; >>>> >>>> memcg = mem_cgroup_iter(NULL, NULL, NULL); >>>> @@ -88,8 +86,6 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v) >>>> } >>>> } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL); >>>> >>>> - rcu_read_unlock(); >>>> - >>>> kfree(count_per_node); >>>> return ret; >>>> } >>>> -- >>>> 2.53.0-Meta >>>> >>> Reviewed-by: Nick Huang >> >> Sorry, but what exactly did you review here? In the past you were >> sending AI slop: >> https://lore.kernel.org/all/aeNGbNyPxJssnkbO@lucifer/ >> >> Not even properly following the process. >> https://lore.kernel.org/all/20260724143408.175870-1-sef1548@gmail.com/ >> >> https://lore.kernel.org/all/?q=f%3Asef1548%40gmail.com >> >> Why suddenly reviewing MM patches? >> >> Best regards, >> Krzysztof > > Hi Krzysztof > > I saw this patch on the mailing list and was interested in the RCU issue, so I looked at the related code. > I checked that removing the outer RCU read-side critical section doesn't break the memcg lifetime, > since mem_cgroup_iter() returns the memcg with a CSS reference held. > I should have explained what I checked instead of just adding a Reviewed-by. > Thanks for pointing that out. After all the maintainers already provided a review... Best regards, Krzysztof