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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 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; keydata= xsFNBFVDQq4BEAC6KeLOfFsAvFMBsrCrJ2bCalhPv5+KQF2PS2+iwZI8BpRZoV+Bd5kWvN79 cFgcqTTuNHjAvxtUG8pQgGTHAObYs6xeYJtjUH0ZX6ndJ33FJYf5V3yXqqjcZ30FgHzJCFUu JMp7PSyMPzpUXfU12yfcRYVEMQrmplNZssmYhiTeVicuOOypWugZKVLGNm0IweVCaZ/DJDIH gNbpvVwjcKYrx85m9cBVEBUGaQP6AT7qlVCkrf50v8bofSIyVa2xmubbAwwFA1oxoOusjPIE J3iadrwpFvsZjF5uHAKS+7wHLoW9hVzOnLbX6ajk5Hf8Pb1m+VH/E8bPBNNYKkfTtypTDUCj NYcd27tjnXfG+SDs/EXNUAIRefCyvaRG7oRYF3Ec+2RgQDRnmmjCjoQNbFrJvJkFHlPeHaeS BosGY+XWKydnmsfY7SSnjAzLUGAFhLd/XDVpb1Een2XucPpKvt9ORF+48gy12FA5GduRLhQU vK4tU7ojoem/G23PcowM1CwPurC8sAVsQb9KmwTGh7rVz3ks3w/zfGBy3+WmLg++C2Wct6nM Pd8/6CBVjEWqD06/RjI2AnjIq5fSEH/BIfXXfC68nMp9BZoy3So4ZsbOlBmtAPvMYX6U8VwD TNeBxJu5Ex0Izf1NV9CzC3nNaFUYOY8KfN01X5SExAoVTr09ewARAQABzSVLcnp5c3p0b2Yg S296bG93c2tpIDxrcnprQGtlcm5lbC5vcmc+wsGPBBMBCgA5AhsDBgsJCAcDAgYVCAIJCgsE FgIDAQIeAQIXgBYhBJvQfg4MUfjVlne3VBuTQ307QWKbBQJp2mE8AAoJEBuTQ307QWKbeaIP /ihHTkTW4KsN/DQ945JJbyu5tI0J80Wue7QyyLPglyKfhgb5cLLNPpOC8cCIJsc7+W3i2P38 s2c1cOH6CYGE7E9ur3Vfme8NW2S2I/Z8VC7bZnzyS23wT17LrsdS/qCpx4o8U+pt/xdXDKph EGRYrIEmMpUWvyYzyYKGIe25FtaayIIKpq8eZYyFcp2f/sG5IkOW5uZzHPMPdcm87jU7fyuQ rAU2vx9r+ulUfQ/q9Z2roC/ode3l7t2pN7BCBCsUDp6JCrUyZrtT1e7EbA0ZRP3aOBNk2P2E DQOgJGjGdO5Yx2Y9LFtltu6JbsBJHi1syGRX3AtQYOMc4Y1WGoeZJmMlvKj2ZqqXNkcWi2DS IQEWB0uW6CqFsBBIMGDa+6OzdaVO/uAVXWDWml02Men3CILdI1MbVjoh8ECqYUY7OQ+JJvNN vnliuq5WM3Ghd3jg/LZZrxXjdIginRHFQCjIJYLKpLZWm1/iDFedcfzqRNYmTtqscdCNHW41 oT3Z7BmO9xwdjuwBS6nmS6JJwkbf5Ot2QR4pB/DRU7ZwjT1qHe+9r9gF32wXVQatHNGK/VVu sfwOnkdxCWkp/qb2gdQRmZh+SedStWshigH6sNfuHBloF/q+hjMRc8b2m326OZdrbSHwY1Sz vti8Hn7n8NjdHO9LKB7BIdjkA9DA5WsqOuVCzsFNBFVDXDQBEADNkrQYSREUL4D3Gws46JEo Z9HEQOKtkrwjrzlw/tCmqVzERRPvz2Xg8n7+HRCrgqnodIYoUh5WsU84N03KlLueMNsWLJBv BaubYN4JuJIdRr4dS4oyF1/fQAQPHh8Thpiz0SAZFx6iWKB7Qrz3OrGCjTPcW6eiOMheesVS 5hxietSmlin+SilmIAPZHx7n242u6kdHOh+/SyLImKn/dh9RzatVpUKbv34eP1wAGldWsRxb f3WP9pFNObSzI/Bo3kA89Xx2rO2roC+Gq4LeHvo7ptzcLcrqaHUAcZ3CgFG88CnA6z6lBZn0 WyewEcPOPdcUB2Q7D/NiUY+HDiV99rAYPJztjeTrBSTnHeSBPb+qn5ZZGQwIdUW9YegxWKvX XHTwB5eMzo/RB6vffwqcnHDoe0q7VgzRRZJwpi6aMIXLfeWZ5Wrwaw2zldFuO4Dt91pFzBSO IpeMtfgb/Pfe/a1WJ/GgaIRIBE+NUqckM+3zJHGmVPqJP/h2Iwv6nw8U+7Yyl6gUBLHFTg2h YnLFJI4Xjg+AX1hHFVKmvl3VBHIsBv0oDcsQWXqY+NaFahT0lRPjYtrTa1v3tem/JoFzZ4B0 p27K+qQCF2R96hVvuEyjzBmdq2esyE6zIqftdo4MOJho8uctOiWbwNNq2U9pPWmu4vXVFBYI GmpyNPYzRm0QPwARAQABwsF2BBgBCgAgAhsMFiEEm9B+DgxR+NWWd7dUG5NDfTtBYpsFAmna YUkACgkQG5NDfTtBYptX+BAApg32CkxwNucNEi8WfWA8oKkW0y8YDuY6ORMo9FWNGiT/OTy0 vyJrLocrpn86zwfjVp+eCrssPYh8eqJfnWqmYv6ACQtHPYzPZQ3mSo8H97Z01oUxITzCxpXm ZkLgPIqtDPcC2E3dPM/fVxcyowM8XsaMA9wcsaUYrta8toOq2b9tKcjleKMfMrm0gQ9u7wUc QbLkwj6TCLOwucb07GXzLTNF9PZmaDUpKAZjMjmrW+le+SFvQbhamx0rxLWPR0NWntXpbCn+ +ACch03p/JyTBVktxFsFyCt7pTPE1kEaeuXBTe/a2D9iQvRxRW19LvuO2e59/u1wYUiH/orz wbIC2S4dBsPAPihL3ztOU1yE86GPyQtSE0kU+/7snnLt4QGi6PChf3t5gnNjAzjUUovO8rgI c+5yN5heq5loYHgK6OQ9OlHzsPHO9e9MOQcKlFycs1pyijFGzDwdNUm/SchK8iWT2QApTx4A K9bCVaboTA2T77QYkRcRJYSsO1alGX0ome/hMLD1daXlkrNUp1HWa3K4iytLRXjCSIorWiGs n+q3krnpXu3TFkA8qtOFZMdnIiFuiq1yLT8hptsV5xh1TA2nsVvSYiaCr3q4s4BKjS/KrLDb qoxzw8ISjdUp4pA85vb6YLCmb39NgidD+7PmAr65lBNveIFynTgsja1rRQ4= In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: 8ymqb5eb3ijkinf8u7y5b8twtd8twfyp X-Rspamd-Queue-Id: B4B1FC0004 X-Rspamd-Server: rspam03 X-Rspam-User: X-HE-Tag: 1787666173-644173 X-HE-Meta: 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 s5VsnOma m1RxPHIlB2QBRDNZGi+hLCq3lC+v3jSTa4pSdUXwqx7gyrEPW0PMdf/SReXlmryG8Qhb8gHBzjz3NR9EJPpO3AjvQN/LySu+xAuoJ/Zc6/ShSE5XcQ3p238UKVcQ/CD9QThwKYrhMUEzJtlXTUfd77ahA4cyjUVkI8u44su7tkmbKU31zwy6pPLNMzSwKctXbU+V5VmubEJqSrSMa8G2deaIzQw0F8nj4yiBu5KdKgS7C57tAuyy7S6FppA78xn6Yi2HdgrFhS2JotnrWqXrWmdRZvsjONjq0XFEI4Ks0Pxuu1KPoHZkRLpo4jJPFMA38xk4ji9BaLQuZqkRSIuAm+wH6J2E325aN69sC Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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