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b=HRPKxo0Rr3pbVCWT//XwS+W4dEPHI6hqcz1BIoaq/8A0qT653i2ccP8D1Qww3C6ky ZHc7iEGKxZLeGyTypJxFtRlnjb/2M3CM0cO28M2puxzKI5v4P4mt6TjFKzFE4BuSSP 3IO6sBs03lpsURbOoVFbQFXSE4G53T0ZCSQqMnfI7+P433zwNCUWHBb0QTdHYJe35W YayvavbbDgufFAFMzARoxcEMsGrUUxi2pIUkGdpv6gts0HDwuhPomEWmife+pLGAB2 i13ajRFu0ZeqIkoJ4iqrcwwv4xJ8mOBrtEYhSQCsxbEIHTCi5bD/4ggAtjwxoJqcPR 5Q8Yh24sykFlQ== Message-ID: <04ea3e05-5b83-4c2d-8759-67c544a7b484@kernel.org> Date: Fri, 26 Jun 2026 10:17:14 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm: annotate data-race in cpu_needs_drain() To: Xuewen Wang , akpm@linux-foundation.org, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, jannh@google.com, pfalcato@suse.de, chrisl@kernel.org, kasong@tencent.com, shikemeng@huaweicloud.com, nphamcs@gmail.com, baoquan.he@linux.dev, baohua@kernel.org, youngjun.park@lge.com, qi.zheng@linux.dev, shakeel.butt@linux.dev, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260626053700.2036899-1-wangxuewen@kylinos.cn> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: abodywqmci3brhhpj71fkoxkot5zb459 X-Rspamd-Queue-Id: 94998100009 X-Rspamd-Server: rspam06 X-HE-Tag: 1782461843-670694 X-HE-Meta: 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 RU2eWuhj EQt0q1E3fa7AJsJMvWLdCrV3QYZsbEmMLZhBj/+D7vay/LL7eAXYF+u5nSME3md97yyQbdPBh6gW6XYzyb+/4SfD35BItLR9sDRQ1dib/HEtg8P7Of6hMCabgNkofjTaPuN/fkqsMwAv2HZR8OoUTnh14zkTjZ3daiIjamTCWDgDR6JewE0JS3IeO89jM1C3Kbza6eTtNMk6O7LZFOLgZvnwTX28vznEp+SAmCVO0T5gR9Gt3PNoheF1sXqDXYLIk3wXrGSpN+M4uYfVTFsoig6HLTu7wL0dUnTaYcn7hIY7zmtlS/RcP5/uMNLg42mJDTi8VldRRAJQy5d4= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/26/26 07:37, Xuewen Wang wrote: > KCSAN reports a data-race when cpu_needs_drain() reads another CPU's > per-cpu folio_batch->nr without locking, while the owning CPU writes > to it via folio_batch_add(). > > Reading a slightly stale value is harmless -- cpu_needs_drain() only > decides whether to schedule a drain, and the next iteration of > __lru_add_drain_all() will re-check. Use data_race() to annotate > the intentional race. > > Signed-off-by: Xuewen Wang > --- > Changes in v3: > - Wrap the entire || expression in a single data_race() instead of wrapping > each folio_batch_count() call individually, as suggested by Pedro and Lorenzo. > This is equally effective and more readable. > - Remove data_race() from need_mlock_drain(), as it is now covered by the data_race() > in cpu_needs_drain(). > v2: > https://lore.kernel.org/all/20260625065153.1581419-1-wangxuewen@kylinos.cn/ > v1: > https://lore.kernel.org/all/20260624092606.1083449-1-wangxuewen@kylinos.cn/ > --- > mm/swap.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/mm/swap.c b/mm/swap.c > index 588f50d8f1a8..46ea207e0624 100644 > --- a/mm/swap.c > +++ b/mm/swap.c > @@ -828,13 +828,13 @@ static bool cpu_needs_drain(unsigned int cpu) > struct cpu_fbatches *fbatches = &per_cpu(cpu_fbatches, cpu); > > /* Check these in order of likelihood that they're not zero */ > - return folio_batch_count(&fbatches->lru_add) || > + return data_race(folio_batch_count(&fbatches->lru_add) || > folio_batch_count(&fbatches->lru_move_tail) || > folio_batch_count(&fbatches->lru_deactivate_file) || > folio_batch_count(&fbatches->lru_deactivate) || > folio_batch_count(&fbatches->lru_lazyfree) || > folio_batch_count(&fbatches->lru_activate) || > - need_mlock_drain(cpu) || > + need_mlock_drain(cpu)) || The indentation is a bit suboptimal now. Would read nicer as diff --git a/mm/swap.c b/mm/swap.c index 588f50d8f1a8c..5958e6fdd3593 100644 --- a/mm/swap.c +++ b/mm/swap.c @@ -828,13 +828,13 @@ static bool cpu_needs_drain(unsigned int cpu) struct cpu_fbatches *fbatches = &per_cpu(cpu_fbatches, cpu); /* Check these in order of likelihood that they're not zero */ - return folio_batch_count(&fbatches->lru_add) || - folio_batch_count(&fbatches->lru_move_tail) || - folio_batch_count(&fbatches->lru_deactivate_file) || - folio_batch_count(&fbatches->lru_deactivate) || - folio_batch_count(&fbatches->lru_lazyfree) || - folio_batch_count(&fbatches->lru_activate) || - need_mlock_drain(cpu) || + return data_race(folio_batch_count(&fbatches->lru_add) || + folio_batch_count(&fbatches->lru_move_tail) || + folio_batch_count(&fbatches->lru_deactivate_file) || + folio_batch_count(&fbatches->lru_deactivate) || + folio_batch_count(&fbatches->lru_lazyfree) || + folio_batch_count(&fbatches->lru_activate) || + need_mlock_drain(cpu)) || has_bh_in_lru(cpu, NULL); } But I'll let others decide :) Acked-by: David Hildenbrand (Arm) -- Cheers, David