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bh=LRHs68uZvA9um2gm65aY4M4ReG5L7r5Gco47C8yFhfw=; b=wiop+6+SPPR19zJRjeyXnyiM1oPRdwt0Xu5+thK2MHtBFRV9+LQ4qYZcRML7FDIrXgkQDH eP8wDl+2L30H73mKe89d2Wxe5ilWWsxmf+SzhQFxjw1P3cyiEmgeLzFfILHh3goqyIwbfc lnh4A79WnDiKenXbjEa+kyCCZL1J6DY= ARC-Authentication-Results: i=1; imf08.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=er20X6E4; spf=pass (imf08.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id A819B60121; Fri, 3 Jul 2026 09:21:03 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B97931F000E9; Fri, 3 Jul 2026 09:20:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783070463; bh=LRHs68uZvA9um2gm65aY4M4ReG5L7r5Gco47C8yFhfw=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=er20X6E4CCctj3eE7ZkCYz1M034Z/dOeLXhhW4SXSHRXrNAdk8ORvRd78IdzAbnlx 1XwS/Sj4Qy41VCSnWZTXn/Ueyhi9Hkl3GhMZyM02Pf2XRW4AoSTrJv/HwmO3VIFhpy H3E9Rxi9sFJ0olWtSmlvlQkcSPXm/DiXePmcUSydvdHxi2IOPNM2VvUO+xmofYmgaG KOBXjNOGM7D4+q538hTKSTBVko3mqvABUa1ZQvMRGWR03r2nnoKNTz6I+m27A6wdZN J7hUAsQXmzayViW3Mv1ZEg2RwRb/9a6JbeIudqEvgrciOYFi/iOHW6gWmPUAqtm8Ly 2Ksvsni8s0uIw== Message-ID: <474e7222-7589-4e8b-8a3c-f85ec2335f19@kernel.org> Date: Fri, 3 Jul 2026 11:20:56 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH stable] mm/khugepaged: write all dirty file folios when collapsing To: Lance Yang Cc: pfalcato@suse.de, akpm@linux-foundation.org, ljs@kernel.org, baolin.wang@linux.alibaba.com, liam@infradead.org, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, stable@vger.kernel.org, viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, willy@infradead.org, song@kernel.org, ehagberg@janestreet.com, ziy@nvidia.com, gleventhal@janestreet.com References: <20260703090232.26261-1-lance.yang@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 X-Rspam-User: X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 36F2A160005 X-Stat-Signature: ihahrk7be61543buia1a5mzt8wz4q44a X-HE-Tag: 1783070464-519418 X-HE-Meta: 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 LyfEGzQI eYE9xYpbaU/gvYt15Ch0JlYzbWwlxZPw3LMkV/ddxs9n+tBh4zhrp42qRoC3y3lMm2cbBnP4XJU7sRwAOWuB5oQQa306/EouW4MuQXg8CKJdWaLzie1eUCEYPgdGjCPFbfShGlbNlEMvZm5IZG1toiyMVD6fQOYP7lNE2soF+en60ZQbE6yr6MyLpelCfokWn5asqFrkrmei8+aryKMEt1S+2BDtOG5TgGSANRkhVu9v80KVNHbwgQAvmYqgKdccaL1EY Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/3/26 11:02, Lance Yang wrote: > > On Fri, Jul 03, 2026 at 10:55:42AM +0200, David Hildenbrand (Arm) wrote: >> On 7/2/26 18:54, Pedro Falcato wrote: >>> As-is, khugepaged and writable-file opening exclude each other. A file >>> cannot be open writeable and have THPs (because the filesystem is not aware >>> of them). khugepaged will never collapse file pages for files that are >>> opened writeable. On an open(O_RDWR/O_WRONLY), the page cache for that >>> particular file is dropped. This is fine because nothing could've been >>> dirtied. >>> >>> However, there is an edge-case: collapse_file() might not be able to >>> coexist with concurrent writers, but it can coexist with dirty folios >>> (from previous writers). Therefore, the following can happen: >>> >>> open(file, O_RDWR) >>> write(file) >>> close(file) >> >> Okay, folios are dirty. >> >>> madvise(file_mapping, MADV_COLLAPSE, some non-dirty range) >> >> collapse_file() has >> >> if (!is_shmem && (folio_test_dirty(folio) || >> folio_test_writeback(folio))) { >> ... >> result = SCAN_PAGE_DIRTY_OR_WRITEBACK; >> goto out_unlock; >> } >> >> Making us abort collapse. >> >> What am I missing? > > Hmm ... dirty folios can be outside the range being collapsed ... > > For example: > > write/dirty: [6M, 8M) > MADV_COLLAPSE: [0M, 2M) > > collapse_file() only checks the folios in the collapse range, so the > dirty/writeback check passes for [0M, 2M). But after that, for the old > READ_ONLY_THP_FOR_FS case, nr_thps gets bumped for the mapping. Ahh, so it's unrelated folios, thanks for clarifying that. ("some non-dirty range" documents that) All makes sense to me, thanks! -- Cheers, David