From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from m16.mail.163.com (m16.mail.163.com [220.197.31.5]) (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 6522F1F1513; Thu, 3 Sep 2026 09:14:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=220.197.31.5 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788426853; cv=none; b=a3H/0mUBVLj3/D5PyUaRCttGTjdz/WzgaXTwC1XrjgmAeR1DtpA9B/3qX5fh1QMOelymfqMHFHJMMbRdmkxFoA6mo6lYh+e7p+/9SE0/+F5IpO8+B03GGKv5+xm/pQYdIZwugCbdT1oKwViYy0dHSvDfIele3N9KWSIvaae2nnM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788426853; c=relaxed/simple; bh=pwxCa+02bNBfjuGiRu7O9G/SCnZvL9HS8DRVX7csQqQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=fly6Bu75u95bQQIlpuj1xSMGjzSo7WDBnVE+2oPp5CwKR8PRWUMKwu+okNiUHQF3yJs7lGNq92LEbzmzAHpcKjr9RdoGe7j+8rwe0Hr0Pftx7IUqfZ+whY8P6a3WUMs96BHVWgOY3rL/O3Ytu+qICNox2Th3vrM437ye0PCgQmQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=163.com; spf=pass smtp.mailfrom=163.com; dkim=pass (1024-bit key) header.d=163.com header.i=@163.com header.b=d1mPkZQO; arc=none smtp.client-ip=220.197.31.5 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=163.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=163.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=163.com header.i=@163.com header.b="d1mPkZQO" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=163.com; s=s110527; h=Message-ID:Date:MIME-Version:Subject:To:From: Content-Type; bh=6Eb7OOCldREczEc1zP4/rwq++5YqOpKv0/oThiUkNrw=; b=d1mPkZQOntVWuJ3jJY+aIT4a0yYdMl2bhSahqBGDPdT3cvnlsYz1PeyUmxmEQL TzfaF5/o5YFGlOKwTqW1zgL/NbC522CD3ybxrokwxf284vI9XLNX6Z0g1CG258ij wJ8/mKoLu/wKqkRuqRQXW1CT8XnDEciKfuegBGzJhsaHA= Message-ID: Date: Thu, 3 Sep 2026 17:13:15 +0800 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 v2] mm/slab_common: fix shrink budget underflow in kfree_rcu_shrink_scan To: Harry Yoo Cc: vbabka@kernel.org, akpm@linux-foundation.org, hao.li@linux.dev, cl@gentwo.org, rientjes@google.com, roman.gushchin@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, paulmck@kernel.org, rcu@vger.kernel.org, xialonglong@kylinos.cn References: <20260826075653.3304251-1-xialonglong2025@163.com> From: Longlong Xia In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-CM-TRANSID:_____wDnt+wqOplqb5p0AQ--.21702S2 X-Coremail-Antispam: 1Uf129KBjvJXoWxJFyrAF4ftF13XFyDGw1UAwb_yoWrZr4rpF W0ka4UKr4kJry8u3srXw4xXryay3yFkFWxGrWDAr43J3sI9F1rtryIyr1qvrWFyrn3Jw4S vr1qgFyDXFyjvFJanT9S1TB71UUUUU7qnTZGkaVYY2UrUUUUjbIjqfuFe4nvWSU5nxnvy2 9KBjDUYxBIdaVFxhVjvjDU0xZFpf9x07U5sqAUUUUU= X-CM-SenderInfo: x0ldz0pqjo00rjsqjki6rwjhhfrp/xtbC3A2ak2qZOi2XJAAA3P Thanks for the reply. 在 2026/8/30 22:22, Harry Yoo 写道: > On Wed, Aug 26, 2026 at 03:56:53PM +0800, Longlong Xia wrote: >> From: Longlong Xia > Hi Longlong, I have a few questions. > >> The kfree_rcu shrinker decremented sc->nr_to_scan (unsigned long) >> and then tested the result with <= 0. When a single CPU's object >> count exceeds the remaining budget, the subtraction wraps to a large >> positive value and the <= 0 comparison, which is equivalent to == 0 >> for an unsigned type, never fires again. > Since when (which commit) has it been broken? > If it has been undiscovered for a very long time, why is that so? Since the shrinker was first added: 9154244c1ab6 ("rcu/tree: Add a shrinker to prevent OOM due to kfree_rcu() batching", v5.8); > > And how did you discover this? While reviewing the kvfree_rcu() batching paths with the help of AI tooling (hence the Assisted-by: tag in the patch), which flagged the unsigned subtraction feeding a "<= 0" test. >> The scan loop then iterates >> through every possible CPU instead of honouring the reclaim budget. > Did you confirm this actually does happen? If so, how often does the > kernel end up iterating through every possible CPUs, very rarely or > almost always? Yes, in two ways. Experiment 1 -- deterministic, instrumented loop.  Per-CPU pr_info() plus a test entry invoking kfree_rcu_shrink_scan() with a controlled budget; 4-CPU QEMU guest, 400 objects queued on CPU0, budget 50:   buggy:   cpu=0 count=400 nr_to_scan_after_sub=18446744073709551266                 visits all 4 CPUs   patched: cpu=0 count=400 nr_to_scan=50 BREAK                stops at CPU0 Experiment 2 -- natural trigger, no kernel modification at all (kprobes only).  Setup: 1 GiB QEMU guest, 4 possible CPUs, no swap.   1. Observation: three kprobes.  Scan entry/exit record nr_to_scan      and the return value; each kfree_rcu_monitor() hit inside a      scan is one CPU the loop processed.   2. Memory pressure: ~650 MiB of anonymous memory.   3. krc backlog: loaded the in-tree test_vmalloc module ,      which repeatedly vmalloc()s one page and frees it via kvfree_rcu();     10 million iterations over ~70s kept the per-CPU queues populated. Result within ~70s: 262 natural invocations, all from kswapd0; 171 of the 261 scans that found a backlog (66%) overshot the budget (nr_to_scan = 128, freed up to 4479) and visited all 4 possible CPUs. > > Would this affect the kernel's reclamation behavior in some way? The break was meant to cap the scan at the reclaim budget; once the subtraction wraps, the cap is gone and one scan processes every possible CPU, draining the whole backlog instead of a budget-sized slice. Thanks, Longlong >> Accumulate into freed and stop once freed >= nr_to_scan. The shrinker >> core treats nr_to_scan as input-only, so dropping the decrement is >> safe; freed becomes unsigned long to match the return type. >> >> Suggested-by: Hao Li >> Assisted-by: Codex:gpt-5.6-sol >> Signed-off-by: Longlong Xia >> --- >> Changes in v2: >> - Rework per suggestion from Hao Li: accumulate into freed directly, >> compare freed >= nr_to_scan instead of decrementing nr_to_scan, and >> drop the per-CPU count local; promote freed to unsigned long. >> >> Link: https://lore.kernel.org/all/20260824091838.1692153-1-xialonglong2025@163.com/ >> --- >> mm/slab_common.c | 13 +++++-------- >> 1 file changed, 5 insertions(+), 8 deletions(-) >> >> diff --git a/mm/slab_common.c b/mm/slab_common.c >> index 657fd75776ea..e227c2ef2a4e 100644 >> --- a/mm/slab_common.c >> +++ b/mm/slab_common.c >> @@ -2162,20 +2162,17 @@ kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc) >> static unsigned long >> kfree_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc) >> { >> - int cpu, freed = 0; >> + int cpu; >> + unsigned long freed = 0; >> >> for_each_possible_cpu(cpu) { >> - int count; >> struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu); >> >> - count = krc_count(krcp); >> - count += drain_page_cache(krcp); >> + freed += krc_count(krcp); >> + freed += drain_page_cache(krcp); >> kfree_rcu_monitor(&krcp->monitor_work.work); >> >> - sc->nr_to_scan -= count; >> - freed += count; >> - >> - if (sc->nr_to_scan <= 0) >> + if (freed >= sc->nr_to_scan) >> break; >> } >> >> -- >> 2.43.0