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Wong" 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: References: <20260309134506.167663-1-bfoster@redhat.com> <20260309134506.167663-9-bfoster@redhat.com> <20260309175602.GR6033@frogsfrogsfrogs> 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: <20260309175602.GR6033@frogsfrogsfrogs> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 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?" > > 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. 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 > > > > >