From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 90EFB3FB051; Mon, 9 Mar 2026 17:56:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773078963; cv=none; b=tXNZHMgj5bEKR7rHxIfjWwoYFkF1zmCYnmCX/9ICkgrUxBoTamsne+N+TM8W3fexOhca/fwkr6EEZGkvX18V0rG8H3feO/AkBt47g+fU1Q5osJUgN9he/ilbequ+KlaYm3YlsalvpaMkcJBCu2vFsyZkXZgRal5trjaU4fY0e/8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773078963; c=relaxed/simple; bh=zqQ1M4c2Y7qiF4Yb6oHDeIKD+YastSuucsI4hR5wB0E=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=U5BipYOtSTZXpgmo1IPgzgysesy6EbLBdPhrmX7xClzzQZnYlMjzji/66oX2MkzFvDU5fmTYehmPHV2ykj9pOBecu44lcQwc1djrUIucUKTkYlsq2v/NtZQ+9mYHm2GSTcu1b1dv3zv9azX58DeSyviFXy52azShuiFZ4bwnRhA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=L1ehr5xu; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="L1ehr5xu" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0D14BC4CEF7; Mon, 9 Mar 2026 17:56:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773078963; bh=zqQ1M4c2Y7qiF4Yb6oHDeIKD+YastSuucsI4hR5wB0E=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=L1ehr5xunsGhI2ul9IZ21ExmgoTTA16YXbGjxk1KGocxdWmGGWa+JEifobDE1t7M2 lOVcaORnIFdytzQpk+5HarNc/39RT9MLKSihrEMTB/x/zVFBW9ChoaDG94spLXyCx7 ubedWUSVsVn18ziraRKzOybizGWrgfYUfbqTiXby9wmjptuupIhaH0lfgVmx35PRtH yqOhxyI3I1Xd3T2i3nq080DPBJQ4as5xvZA/bmNOct0jhjmbqjxBKo75GGlR3BQ9w3 +QFFevzGa7IF1sLVd6ZsM58KjSAlllfC35QXfvaahsD0zMsiIK0Fa9GxJjPPkU3qIN dez6HU0d9CPKg== Date: Mon, 9 Mar 2026 10:56:02 -0700 From: "Darrick J. Wong" To: Brian Foster Cc: linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org Subject: Re: [PATCH v3 8/8] xfs: report cow mappings with dirty pagecache for iomap zero range Message-ID: <20260309175602.GR6033@frogsfrogsfrogs> References: <20260309134506.167663-1-bfoster@redhat.com> <20260309134506.167663-9-bfoster@redhat.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260309134506.167663-9-bfoster@redhat.com> On Mon, Mar 09, 2026 at 09:45:06AM -0400, Brian Foster wrote: > XFS has long supported the case where it is possible to have dirty > data in pagecache backed by COW fork blocks and a hole in the data > fork. This occurs for two reasons. On reflink enabled files, COW > fork blocks are allocated with preallocation to help avoid > fragmention. Second, if a mapping lookup for a write finds blocks in > the COW fork, it consumes those blocks unconditionally. This might > mean that COW fork blocks are backed by non-shared blocks or even a > hole in the data fork, both of which are perfectly fine. > > This leaves an odd corner case for zero range, however, because it > needs to distinguish between ranges that are sparse and thus do not > require zeroing and those that are not. A range backed by COW fork > blocks and a data fork hole might either be a legitimate hole in the > file or a range with pending buffered writes that will be written > back (which will remap COW fork blocks into the data fork). > > This "COW fork blocks over data fork hole" situation has > historically been reported as a hole to iomap, which then has grown > a flush hack as a workaround to ensure zeroing occurs correctly. Now > that this has been lifted into the filesystem and replaced by the > dirty folio lookup mechanism, we can do better and use the pagecache > state to decide how to report the mapping. If a COW fork range > exists with dirty folios in cache, then report a typical shared > mapping. If the range is clean in cache, then we can consider the > COW blocks preallocation and call it a hole. > > This doesn't fundamentally change behavior, but makes mapping > reporting more accurate. Note that this does require splitting > across the EOF boundary (similar to normal zero range) to ensure we > don't spuriously perform post-eof zeroing. iomap will warn about > zeroing beyond EOF because folios beyond i_size may not be written > back. Hrmm. I wonder if IOMAP_REPORT should grow this new "expose dirty unwritten cow fork mappings over a data fork hole" behavior too? I guess the only user of IOMAP_REPORT that might care is swapfile activation, but that fsyncs the whole file to disk before starting the iteration so I think it won't matter? > Signed-off-by: Brian Foster /me isn't sure he sees the point of doing this only for IOMAP_ZERO but you're right that it's weird to pass a folio batch and a hole mapping to iomap so Reviewed-by: "Darrick J. Wong" --D > --- > fs/xfs/xfs_iomap.c | 25 +++++++++++++++++++++---- > 1 file changed, 21 insertions(+), 4 deletions(-) > > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c > index df240931f07a..3bef5ea610bb 100644 > --- a/fs/xfs/xfs_iomap.c > +++ b/fs/xfs/xfs_iomap.c > @@ -1786,6 +1786,7 @@ xfs_buffered_write_iomap_begin( > xfs_fileoff_t offset_fsb = XFS_B_TO_FSBT(mp, offset); > xfs_fileoff_t end_fsb = xfs_iomap_end_fsb(mp, offset, count); > xfs_fileoff_t cow_fsb = NULLFILEOFF; > + xfs_fileoff_t eof_fsb = XFS_B_TO_FSB(mp, XFS_ISIZE(ip)); > struct xfs_bmbt_irec imap, cmap; > struct xfs_iext_cursor icur, ccur; > xfs_fsblock_t prealloc_blocks = 0; > @@ -1868,7 +1869,8 @@ xfs_buffered_write_iomap_begin( > * cache and fill the iomap batch with folios that need zeroing. > */ > if ((flags & IOMAP_ZERO) && imap.br_startoff > offset_fsb) { > - loff_t start, end; > + loff_t start, end; > + unsigned int fbatch_count; > > imap.br_blockcount = imap.br_startoff - offset_fsb; > imap.br_startoff = offset_fsb; > @@ -1883,15 +1885,32 @@ xfs_buffered_write_iomap_begin( > goto found_imap; > } > > + /* no zeroing beyond eof, so split at the boundary */ > + if (offset_fsb >= eof_fsb) > + goto found_imap; > + if (offset_fsb < eof_fsb && end_fsb > eof_fsb) > + xfs_trim_extent(&imap, offset_fsb, eof_fsb - offset_fsb); > + > /* COW fork blocks overlap the hole */ > xfs_trim_extent(&imap, offset_fsb, > cmap.br_startoff + cmap.br_blockcount - offset_fsb); > start = XFS_FSB_TO_B(mp, imap.br_startoff); > end = XFS_FSB_TO_B(mp, imap.br_startoff + imap.br_blockcount); > - iomap_fill_dirty_folios(iter, &start, end, &iomap_flags); > + fbatch_count = iomap_fill_dirty_folios(iter, &start, end, > + &iomap_flags); > xfs_trim_extent(&imap, offset_fsb, > XFS_B_TO_FSB(mp, start) - offset_fsb); > > + /* > + * Report the COW mapping if we have folios to zero. Otherwise > + * ignore the COW blocks as preallocation and report a hole. > + */ > + if (fbatch_count) { > + xfs_trim_extent(&cmap, imap.br_startoff, > + imap.br_blockcount); > + imap.br_startoff = end_fsb; /* fake hole */ > + goto found_cow; > + } > goto found_imap; > } > > @@ -1901,8 +1920,6 @@ xfs_buffered_write_iomap_begin( > * unwritten extent. > */ > if (flags & IOMAP_ZERO) { > - xfs_fileoff_t eof_fsb = XFS_B_TO_FSB(mp, XFS_ISIZE(ip)); > - > if (isnullstartblock(imap.br_startblock) && > offset_fsb >= eof_fsb) > goto convert_delay; > -- > 2.52.0 > >