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 EA2064A35 for ; Thu, 11 Jun 2026 00:22:51 +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=1781137372; cv=none; b=OWPvk7LGvOupm3yAv0+rIhh9ovECOG15e8/Mxeq+yvqxhrgU5OAQ+RfCAeM9IddKiAB1ExNToZ10Z+uceJEQ0BY/k3tUGwHBjQCwArL8hoRcOrXkgIWo+qwzDhY54CdwMmpvguaP9D6avdPkmyxyiDpiI6W1zvKbGsS32QlsW8E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781137372; c=relaxed/simple; bh=gktrFOHDr5d3OEGLrEfiP9BIt0tTvmm8KtApPtyPuX8=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=i4CfrfcLIvZ8XsaukHyVn0gtlFwEKPhoXkXbEkiZsrwgsNhj2xZBdmvgD0r/SSv+Vl0PhyztalFDb3QItqkMFBB7Gf2WLMY2vl1ow0R1CMJbrcf2E6gXmZoBa5bHX+JarHLRBW82Lo5Eois/gcsdMNBOc6JIO6FYgNusnpAp97M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Lxbh7HPQ; 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="Lxbh7HPQ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 27B281F00893; Thu, 11 Jun 2026 00:22:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781137371; bh=2SvRs21mGVxCJSkrSt9EaDORfooGulFSzr+iBZ0ww28=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=Lxbh7HPQ7YCt/Q+bwgr4myrYnjUXT7Vo/3fMu/0XWpHdBnxLX3Dc45N+I1GmceUWp 7/7xSwT4hduI8gyQ4Y9aQ6+/VOpTM6qwz+mePKALM6yp31yBR3a7EFOjr1ojweFHOR NfrkaKUvcLGw/Quz1KS1SlLE8yDMtoGstP4K/kKcalN9kKHzOyMmjaaTiHl2IS6Avj GtA0r49DlModp3BIHRRYqyK+r0uDE9Ri9UL8KO+nROhXXi+Sv34OkStKqPC+PD6r4w G/Xc9Sl4IzGHM4RZ91PEYA65f4eJX+mIl2V16z7lpDcMhFMcEOeieXhtSEKmgVuWQE GVdaq8jEQ4Hvg== From: SeongJae Park To: Shakeel Butt Cc: SeongJae Park , 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 Subject: Re: [PATCH] mm/shrinker: do not hold RCU lock in shrinker_debugfs_count_show() Date: Wed, 10 Jun 2026 17:22:35 -0700 Message-ID: <20260611002236.69207-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260610232048.62930-1-shakeel.butt@linux.dev> References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Wed, 10 Jun 2026 16:20:48 -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(). All make sense to me, thank you for the nice description and the fix! > > 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 Reviewed-by: SeongJae Park Thanks, SJ [...]