From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794245409; b=37FvDr0zXG4z69Yt41wP3p7kIkWMhTWyHMSIfSSofGINkkSWO4PQiqzlEBdS+mSY0QBe KdnmC6yhOUYalQS8S2qT6J95we+hk0w0fZ6JVzh/48FIF/sMekVtzLbTepXXHypqAY8c0 NJ66x4y+pLQYbp/8YMEVcXUkAAoCArGDjfLB1dx7R4mAs/7CvKqi9Nxo6XiIWoi4ptG84 D60pRm46P7jAkAAz2bRMDGGTziWZ7L6X9cxwGcGSqAPPmPx4DNFfPg2R2htbzox8+1m03 RSXbkqnnqgDTrriW7GEAfx9YC6+7nikkGuDg+MlIS1WsBHzQaeNUvxGtUvuhvGOs5uA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794245409; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=8ZMtATJ/UU7bDo4o1x8DtF/SV0R6ntQW02njcbLrqVg=; b=xsKRPAj2VUHYhBdAHp5BKuFV8KgbGFNSkuTwPPZMqVeQaB5LzPQzEsysoE6jmwqWmAgg XP0nkskM3f8C+0ORaCmSDQ3KW97gjv4X3p6noZr3vUV1Eqibzqcq+NYbI4QNeXEM6aY+A CGGO92wLzPU0vTkHRwks3mFNs0Q6WvejX6fkeZQWLE52m+S5HAQkiDeln3BcZU7PhYRvk C7EM426r2xsBOjjB7fLe/rbW9Rp7ADEnyrAl2MZOEF5Ajx2ViwECAQeC7WeuHAJ9cMOEv RAl34mEy8tCtR0VWsqI+eidXvV4yzseblbxI9f4t80pcT4Uwi02zzvdB8nDaHWBsJxA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=U+p2wwv4; 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 0251E606A31C; Mon, 9 Nov 2026 17:30:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=U+p2wwv4 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4B38E1F0089F; Mon, 9 Nov 2026 17:30:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794245408; bh=8ZMtATJ/UU7bDo4o1x8DtF/SV0R6ntQW02njcbLrqVg=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=U+p2wwv4C+kQrVcwZvssLOeRpnAgXsEULsPEgNyaaaIxvwy/8/VtD7V4xk8S90u/q LOzpzC2I2lypdwvarPxFoXZ7SYIzpf7KZZhW/8vXrIgqI+IgENJqFXQU1wqLD0ZOJt ad9lrl+xLEN7TPiZR3cdsFR1ipJLh+td95pCLRukwR/oOEuLFRXdiRI4ocrMdAlKbR oe6DlyKpJmPXb+qNL3Tk1lYBmV8tdok1UwWiuymyAA+cpLqFduKw2owvoWvPHPaF/3 o+VT27eAshKjqwOUMveGvqJzOJSm4cz5J+3aog8pZOgcSCH8Hr8iwXzAwL8qFGDu3a zpuGkCZ3Ayn/A== Date: Mon, 9 Nov 2026 17:30:02 +0000 From: "Lorenzo Stoakes (ARM)" To: Kiryl Shutsemau Cc: Andrew Morton , "David Hildenbrand (Arm)" , Pedro Falcato , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , "Matthew Wilcox (Oracle)" , Hugh Dickins , Chris Mason , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, Sashiko Subject: Re: [PATCH v2 1/2] mm/memory: recheck PG_uptodate after ->page_mkwrite() Message-ID: References: <20261109120403.82361-1-kas@kernel.org> <20261109120403.82361-2-kas@kernel.org> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Nov 09, 2026 at 05:17:03PM +0000, Kiryl Shutsemau wrote: > On Mon, Nov 09, 2026 at 04:11:40PM +0000, Lorenzo Stoakes (ARM) wrote: > > On Mon, Nov 09, 2026 at 12:04:01PM +0000, Kiryl Shutsemau (Meta) wrote: > > > do_shared_fault() unlocks the folio around ->page_mkwrite(). Writeback > > > can lock it in that window, fail, and clear PG_uptodate with the folio > > > left in the page cache. ecryptfs_writepages() does this when encryption > > > fails. filemap_page_mkwrite() relocks the folio and checks only > > > ->mapping, so the fault maps a folio that is not uptodate: > > > > > > CPU0 CPU1 > > > do_shared_fault() > > > folio_unlock() > > > ecryptfs_writepages() > > > folio_clear_uptodate() > > > filemap_page_mkwrite() > > > folio_lock() > > > finish_fault() > > > > > > The next read(2) or fault on the folio reads it in again from the file, > > > over anything stored through the mapping in the meantime. Those stores > > > are lost. > > > > > > Recheck PG_uptodate once ->page_mkwrite() returns with the folio locked, > > > and retry the fault if it is clear, the same way filemap_fault() retries > > > on a cached folio that lost PG_uptodate. Skip folios outside the page > > > cache: driver pages from ->fault() never get PG_uptodate and would > > > retry forever. > > > > > > The check stays out of do_page_mkwrite(): wp_page_shared() calls it on > > > a folio that is still mapped, and a retry there would fault on the same > > > PTE forever. > > > > > > Reported-by: Sashiko > > > Closes: https://lore.kernel.org/sashiko-outbox-202966@kernel.org/ > > > > No Fixes? Sorry I may have missed context from v1 here :) > > The window is from 2007, when __do_fault() started to drop the folio > lock around ->page_mkwrite(): > > Fixes: 6967614761fd ("ocfs2: release page lock before calling ->page_mkwrite") Yeah not insisting! > > Not sure how useful such Fixes is. It predates any possible stable tree. > > > > --- > > > mm/memory.c | 22 ++++++++++++++++++++++ > > > 1 file changed, 22 insertions(+) > > > > > > diff --git a/mm/memory.c b/mm/memory.c > > > index fe72e66c9c2e..cc1d0b7d15c7 100644 > > > --- a/mm/memory.c > > > +++ b/mm/memory.c > > > @@ -5613,6 +5613,18 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf) > > > return VM_FAULT_OOM; > > > } > > > > > > +/* > > > + * A page cache folio that is not uptodate holds data that was never read in > > > + * or cleared, and mapping it leaks stale memory to userspace. > > > + */ > > > +static bool file_folio_not_uptodate(struct folio *folio) > > > +{ > > > + if (likely(folio_test_uptodate(folio))) > > > + return false; > > > + /* Driver pages from ->fault() never get PG_uptodate */ > > > > I guess you mean drivers that return non-page cache folios from ->fault()? > > Yes. How about: > > /* > * ->fault() can also return driver memory that is not in the page > * cache. It never gets PG_uptodate. > */ > > I would rather avoid "rmappable". These folios do get > folio_add_file_rmap_ptes() in set_pte_range(), and the word reads like > PG_large_rmappable. As you said, DAX never reaches the check, so "page > cache" is accurate. OK, though my concern is you're making an assumption based on where this is called from, somebody else could use the function somewhere else and... :) But this is nitpicking really, that's fine! > > > > + return !folio_test_anon(folio) && folio->mapping; > > > > Technically folio_test_anon() can never return true based on where this is > > called from right? Could this just be a: > > > > VM_WARN_ON_ONCE(folio_test_anon(folio)); > > /* ... comment */ > > return folio->mapping; > > > > Instead? > > Right, all callers pass a file folio. > > I will make it an assert, but at the top of the helper, so it checks > every call and not only the rare !uptodate one. Thanks! > > > > @@ -6163,6 +6175,16 @@ static vm_fault_t do_shared_fault(struct vm_fault *vmf) > > > folio_put(folio); > > > return tmp; > > > } > > > + > > > + /* > > > + * The folio was unlocked for ->page_mkwrite(). If it lost > > > + * PG_uptodate meanwhile, retry the fault to read it in again. > > > + */ > > > + if (unlikely(file_folio_not_uptodate(folio))) { > > > > As Pedro said, I think this unlikely() is probably unnecessary given the true > > return case is implicitly unlikely. > > As I told Pedro, I prefer it at the call site. That is where the reader > needs to see that the retry is the rare path. But if you insist... You could also remove the likely() in the function itself ;) > > -- > Kiryl Shutsemau / Kirill A. Shutemov -- Cheers, Lorenzo