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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=kraS3KWc; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D7F2C606A31A; Thu, 29 Oct 2026 11:03:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=kraS3KWc Received: by smtp.kernel.org (Postfix) with ESMTPSA id E871A1F01825; Thu, 29 Oct 2026 11:02:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793271789; bh=MN2H/Sd0YoWD7yOsUu+/3nrXR4+HLFAIs61hYMcDiRg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=kraS3KWcR1HapYe2L8S04iaBUBqNCB8CgKH3Z4hF8TMnrmxZGGmnZ2QpTqjbmPO7b mVvrwquiFMJDE9p3UwAnEs+1js4PqXm/tZQtza0S3ZtIpsLWkdtViZEFp6ZJ8SEQST v+nIgqZXH8hsCK1nprjCVjMvlhvvsz91WUv0cl4n2JQlB+wnm6BaRxa/js3GiGpLr3 jizIWEtIUQzmXO+462nKgWuYKe2pZMOfnpo9WCtr1L4+fxPj7YKX4aqbouQaA2aRux vUEyglwL6d5E0toW8MCroM61RUoU3Lux0qi7OwzKJW51rs80KLz2s0rbV7ZIljKCo+ K3ZqDZO9lh7XA== Message-ID: Date: Thu, 29 Oct 2026 12:02:48 +0100 X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 33/46] mm/madvise: fix data loss from MADV_FREE on a KSM page To: xu.xin@linux.dev Cc: akpm@linux-foundation.org, aneesh.kumar@kernel.org, apopple@nvidia.com, axelrasmussen@google.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, baoquan.he@linux.dev, chengming.zhou@linux.dev, chrisl@kernel.org, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, harry@kernel.org, hughd@google.com, jack@suse.cz, jannh@google.com, jgg@ziepe.ca, jhubbard@nvidia.com, josef@toxicpanda.com, joshua.hahnjy@gmail.com, kas@kernel.org, kasong@tencent.com, kees@kernel.org, kexec@lists.infradead.org, lance.yang@linux.dev, liam@infradead.org, linux-arch@vger.kernel.org, linux-cxl@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ljs@kernel.org, matthew.brost@intel.com, mhocko@kernel.org, mhocko@suse.com, minchan@kernel.org, muchun.song@linux.dev, nico.pache@linux.dev, nphamcs@gmail.com, npiggin@gmail.com, osalvador@suse.de, pasha.tatashin@soleen.com, peterx@redhat.com, peterz@infradead.org, pfalcato@suse.de, pratyush@kernel.org, qi.zheng@linux.dev, rakie.kim@sk.com, riel@surriel.com, rppt@kernel.org, ryan.roberts@arm.com, shakeel.butt@linux.dev, shikemeng@huaweicloud.com, stable@vger.kernel.org, surenb@google.com, usama.arif@linux.dev, vbabka@kernel.org, weixugc@google.com, will@kernel.org, willy@infradead.org, ying.huang@linux.alibaba.com, youngjun.park@lge.com, yuanchu@google.com, ziy@nvidia.com References: <20261027-fixes_core-v1-33-6f74bec9a5bb@kernel.org> <20261029045307.131675-1-xu.xin@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/29/26 05:53, xu.xin@linux.dev wrote: >> MADV_FREE can clear the dirty flag of a singly mapped KSM folio. The >> folio remains in the stable tree and can gain new mappings without being >> dirtied again. Once marked lazyfree, reclaim can discard data that those >> mappings still need. >> >> KSM always marks a folio dirty when converting it to a KSM folio in >> try_to_merge_one_page(). Marking a folio lazyfree clears PG_swapbacked >> but leaves PG_dirty unchanged. Preserving PG_dirty makes reclaim restore >> PG_swapbacked instead of discarding the folio. This also protects folios >> that were already lazyfree when KSM converted them. >> >> Skip KSM folios before clearing their dirty flag, checking under the >> folio lock to serialize with KSM conversion. >> >> Fixes: 854e9ed09ded ("mm: support madvise(MADV_FREE)") >> Cc: stable@vger.kernel.org >> Reported-by: Josef Bacik >> Reviewed-by: Lorenzo Stoakes (ARM) >> Signed-off-by: David Hildenbrand (Arm) >> --- >> mm/madvise.c | 9 +++++++++ >> 1 file changed, 9 insertions(+) >> >> diff --git a/mm/madvise.c b/mm/madvise.c >> index 75b409e97dbf2..a16e3cb256b7b 100644 >> --- a/mm/madvise.c >> +++ b/mm/madvise.c >> @@ -756,6 +756,15 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr, >> if (folio_test_swapcache(folio) || folio_test_dirty(folio)) { >> if (!folio_trylock(folio)) >> continue; >> + /* >> + * KSM folios can gain new mappings at any time, so we must >> + * not clear their dirty flag. >> + */ >> + if (folio_test_ksm(folio)) { >> + folio_unlock(folio); >> + continue; >> + } >> + >> /* >> * If we have a large folio at this point, we know it is >> * fully mapped so if its mapcount is the same as its >> >> -- >> 2.43.0 > > The fix looks correct to me: checking folio_test_ksm() under the > folio lock properly serializes with KSM conversion, and skipping > the folio avoids clearing PG_dirty on a KSM folio. Yes. And doing it under the lock is something the original AI fix missed. Thanks! -- Cheers, David