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Wong" Cc: linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org Subject: Re: [PATCH v3 4/8] xfs: flush eof folio before insert range size update Message-ID: References: <20260309134506.167663-1-bfoster@redhat.com> <20260309134506.167663-5-bfoster@redhat.com> <20260309173213.GN6033@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: <20260309173213.GN6033@frogsfrogsfrogs> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 On Mon, Mar 09, 2026 at 10:32:13AM -0700, Darrick J. Wong wrote: > On Mon, Mar 09, 2026 at 09:45:02AM -0400, Brian Foster wrote: > > The flush in xfs_buffered_write_iomap_begin() for zero range over a > > data fork hole fronted by COW fork prealloc is primarily designed to > > provide correct zeroing behavior in particular pagecache conditions. > > As it turns out, this also partially masks some odd behavior in > > insert range (via zero range via setattr). > > > > Insert range bumps i_size the length of the new range, flushes, > > unmaps pagecache and cancels COW prealloc, and then right shifts > > extents from the end of the file back to the target offset of the > > insert. Since the i_size update occurs before the pagecache flush, > > this creates a transient situation where writeback around EOF can > > behave differently. > > > > This appears to be corner case situation, but if happens to be > > fronted by COW fork speculative preallocation and a large, dirty > > folio that contains at least one full COW block beyond EOF, the > > How do we get a large dirty folio with at least one full cow block > beyond i_size? If we did a pagecache write to the file, then at least > the incore isize should have been boosted out far enough that the block > will now be inside EOF, right? > It's been quite some time since I first reproduced and diagnosed this so I'm going from memory, but IIRC it was some odd case like a failure to split a large folio fully within EOF on a truncate down across it due to being dirty. I originally thought the large folio thing was actually the issue, but I don't recall seeing anything that prevents it in this particular situation. So my understanding from that is that it's more unexpected in that we wouldn't create a large folio directly in this situation, but it's still technically possible through oddball fsx sequences. >From there, we can run into the insert situation described above where the i_size update -> flush -> extent shift behavior creates a transient situation where a post-eof block is temporarily within eof. Brian > --D > > > writeback after i_size is bumped may remap that COW fork block into > > the data fork within EOF. The block is zeroed and then shifted back > > out to post-eof, but this is unexpected in that it leads to a > > written post-eof data fork block. This can cause a zero range > > warning on a subsequent size extension, because we should never find > > blocks that require physical zeroing beyond i_size. > > > > To avoid this quirk, flush the EOF folio before the i_size update > > during insert range. The entire range will be flushed, unmapped and > > invalidated anyways, so this should be relatively unnoticeable. > > > > Signed-off-by: Brian Foster > > Reviewed-by: Christoph Hellwig > > --- > > fs/xfs/xfs_file.c | 17 +++++++++++++++++ > > 1 file changed, 17 insertions(+) > > > > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c > > index 6246f34df9fd..48d812b99282 100644 > > --- a/fs/xfs/xfs_file.c > > +++ b/fs/xfs/xfs_file.c > > @@ -1263,6 +1263,23 @@ xfs_falloc_insert_range( > > if (offset >= isize) > > return -EINVAL; > > > > + /* > > + * Let writeback clean up EOF folio state before we bump i_size. The > > + * insert flushes before it starts shifting and under certain > > + * circumstances we can write back blocks that should technically be > > + * considered post-eof (and thus should not be submitted for writeback). > > + * > > + * For example, a large, dirty folio that spans EOF and is backed by > > + * post-eof COW fork preallocation can cause block remap into the data > > + * fork. This shifts back out beyond EOF, but creates an expectedly > > + * written post-eof block. The insert is going to flush, unmap and > > + * cancel prealloc across this whole range, so flush EOF now before we > > + * bump i_size to provide consistent behavior. > > + */ > > + error = filemap_write_and_wait_range(inode->i_mapping, isize, isize); > > + if (error) > > + return error; > > + > > error = xfs_falloc_setsize(file, isize + len); > > if (error) > > return error; > > -- > > 2.52.0 > > > > >