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Fri, 3 Jul 2026 09:18:42 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id p0aMDXJ+R2pBZwAAD6G6ig (envelope-from ); Fri, 03 Jul 2026 09:18:42 +0000 Date: Fri, 3 Jul 2026 10:18:40 +0100 From: Pedro Falcato To: Lance Yang Cc: akpm@linux-foundation.org, david@kernel.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 Subject: Re: [PATCH stable] mm/khugepaged: write all dirty file folios when collapsing Message-ID: References: <20260702165409.164568-1-pfalcato@suse.de> <20260703051129.88453-1-lance.yang@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260703051129.88453-1-lance.yang@linux.dev> X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 7B35C40009 X-Rspam-User: X-Stat-Signature: ocjhar3hsoxeki9w1ju1pne6cotx1miw X-HE-Tag: 1783070326-371214 X-HE-Meta: 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 t2R/bSWr 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Jul 03, 2026 at 01:11:29PM +0800, Lance Yang wrote: > > On Thu, Jul 02, 2026 at 05:54:09PM +0100, 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) > >madvise(file_mapping, MADV_COLLAPSE, some non-dirty range) > >open(file, O_RDWR) > > nr_thps > 0 > > truncate_inode_pages() > > /* THPs are cleared out, but so are the dirty folios */ > > > >When this edge-case happens, there is data loss, as the dirty folios are > >fully discarded. > > Well spotted, thanks! Well, Gregg deserves a lot of the credit :) > > > > >Fix it by fully writing back the page cache (and waiting) when collapsing > >file THPs. Doing so provides the guarantee that no dirty folio will be > >observed while there are active THPs. To fully ensure this is safe, the > >invalidate_lock needs to be held while doing the writeout, so that > >do_dentry_open()'s page cache truncation excludes this write-and-wait. > > > >Cc: stable@vger.kernel.org > >Cc: Alexander Viro > >Cc: Christian Brauner > >Cc: Jan Kara > >Cc: Matthew Wilcox > >Cc: Song Liu > >Cc: Eric Hagberg > >Cc: Zi Yan > >Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS") > >Reported-by: Gregg Leventhal > >Closes: https://lore.kernel.org/linux-mm/CAFN_u7H_0ECF3jixP=T=U7AH5=Q3wQNvJMo8an3VqUDMerQfUw@mail.gmail.com/ > >Tested-by: Zi Yan > >Signed-off-by: Pedro Falcato > >--- > > Tested on v7.1.2. I no longer see the data loss with this patch applied. > > Tested-by: Lance Yang Thanks! -- Pedro