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Received: from willy by casper.infradead.org with local (Exim 4.99.1 #2 (Red Hat Linux)) id 1wednd-00000005PXP-00Z1; Tue, 30 Jun 2026 19:07:13 +0000 Date: Tue, 30 Jun 2026 20:07:12 +0100 From: Matthew Wilcox To: Zi Yan Cc: Gregg Leventhal , "To: Alexander Viro" , Christian Brauner , "Cc: Jan Kara" , Andrew Morton , Song Liu , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Eric Hagberg Subject: Re: Subject: [BUG/RFC] write-open file THP cache purge can discard dirty page cache Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 748A640011 X-Rspam-User: X-Stat-Signature: rkuspk99xwxdh1tcrnugyj8onzan5hts X-HE-Tag: 1782846437-43771 X-HE-Meta: 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 PV+BXRcn sQ4TrT1dEJD7MLiwkCoXi+Gn5ycESpemTIJ1NeBOJe2wbex+5J/eldzH7kgPo6d8mjP5ZoP1HbxJ0uIJ6URtYZaw3Xh7bvuuJ+1UTswR78lIRfThk4vtgbU4DlL9NnftcXUEZnXsHLLEv/Po2VilJq2aNDoZLtBDxRiKHPFWg9iP7b0nVATUKB+A8s5boSm6atcGOINw1Nj3c3dpalA3DzivJvS8faXHd9qJtvAjdyCzRj+9r9G9NnldnNPLerFnMv9oXZ137WMVCuUjZtvTAykVA8IRKMZEk4ekXOXc7QYymIJ8= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Jun 30, 2026 at 03:05:16PM -0400, Zi Yan wrote: > On Tue Jun 30, 2026 at 2:36 PM EDT, Matthew Wilcox wrote: > > On Tue, Jun 30, 2026 at 01:01:53PM -0400, Gregg Leventhal wrote: > >> On an affected 6.12 kernel with CONFIG_READ_ONLY_THP_FOR_FS=y, a file can > >> contain read-only file THPs installed by khugepaged / MADV_COLLAPSE. When that > >> same file is later opened for write, do_dentry_open() notices > >> filemap_nr_thps() and drops the page cache: > > [...] > >> > >> This is unsafe if the mapping also contains dirty folios. > > > > But there shouldn't be any. It should not be possible to have > > dirty folios and THPs in the same file unless the filesystem > > supports large folios natively. > > > > If the file is open for writing, the attempt to create THPs should fail. > > Maybe he means order-0 dirty folios? Even then. They're all supposed to be gone. I don't want to guarantee that the filesystem will properly skip over clean folios during writeback.