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 0694435F5F1; Mon, 9 Mar 2026 18:38:08 +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=1773081489; cv=none; b=Ov12Cc+R+p/7Mr5jsMHQp5s6ATd1mWpJjQxgn/WwZiELHA+UfDxu9fiCIRJIdW9LJYiLrAZDFvUKmF80o9e6xp2aXIRvX3Dv2wZxvIHArmOw/sQJF3OLgjF8B9Fnq8E1rI61Vm3h0CHpwVJqFI42IQvSPQu6ixIEgdyH6jP7Z5I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773081489; c=relaxed/simple; bh=i+xt71aHCPOgKUuRNulfHzWARkqVhXEZq8CwSrmLxH8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=oXCrLsHmmJrM4QfwGwhm/l4Kv5clXG9wDden16zcuG1jHMqfUDRIvhtGOvkyyMqU55o5WBSZc/vY1UKGwyVTpZnhptv4uLdSWc6EzfXrbolPtYyZZFclJh/gff4EFClPwo467BENg/1APyXq1xo6Fw3bXwcw12JUesJCs+hWNt4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=VehPeTwg; 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="VehPeTwg" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 80A3AC4CEF7; Mon, 9 Mar 2026 18:38:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773081488; bh=i+xt71aHCPOgKUuRNulfHzWARkqVhXEZq8CwSrmLxH8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=VehPeTwgxXuwTsGV8f3jkcKQMk3hh6HObfWxxYl3vML2RyqHQcRFlX3HoPhevchaX McUUwMHuCpnBI6/OKqje2FqBrVJCogHY74oT5eVF0fXmlp9moj0n/STeE9/y7w7GE0 4+AIUimk6hIbBQhfQnGi06uXFHHFkHsiXQnBEDghzRPfU9kpOcWnhT45nmOSqfED4/ Jr2PDpVuqlnlEQuDvnNFhllOl/6na7HP7MhDHrK9PpTjHxG465QbedBpjGVRYWUyQk 6d6E01cIon1jH/WN932Jf0HoagCVcKNzx/DdH12O3Qfqe9qRadOpFY8Zih/DBxeMu1 om97cmvOwtmIQ== Date: Mon, 9 Mar 2026 11:38:08 -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: <20260309183808.GS6033@frogsfrogsfrogs> References: <20260309134506.167663-1-bfoster@redhat.com> <20260309134506.167663-9-bfoster@redhat.com> <20260309175602.GR6033@frogsfrogsfrogs> Precedence: bulk X-Mailing-List: linux-xfs@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: On Mon, Mar 09, 2026 at 02:31:28PM -0400, Brian Foster wrote: > On Mon, Mar 09, 2026 at 10:56:02AM -0700, Darrick J. Wong wrote: > > 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? > > > > I'd have to take a closer look at that and some of the other iomap ops. > I had similar thoughts in the past about whether this might help clean > up seek hole/data and whatnot as well. For here it's primarily just a > cleanup, but IMO it's better for iomap if it doesn't have to carry the > caveat of "is this hole really a hole?" Ooooh, right -- that does need to get fixed for SEEK_{DATA,HOLE} because otherwise it'll miss dirty data over a cow preallocation. > > > 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" > > > > Thanks. NP. :) --D > Brian > > > --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 > > > > > > > > > >