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Wong" Cc: linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org, linux-mm@kvack.org, hch@infradead.org, willy@infradead.org Subject: Re: [PATCH v3 6/7] iomap: remove old partial eof zeroing optimization Message-ID: References: <20250714204122.349582-1-bfoster@redhat.com> <20250714204122.349582-7-bfoster@redhat.com> <20250715053417.GR2672049@frogsfrogsfrogs> <20250715143733.GO2672029@frogsfrogsfrogs> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250715143733.GO2672029@frogsfrogsfrogs> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 X-Rspam-User: X-Rspamd-Queue-Id: 5653010000F X-Rspamd-Server: rspam06 X-Stat-Signature: nid6mi6513xfit8ukzwh8p1q38osr8f4 X-HE-Tag: 1752596202-404049 X-HE-Meta: 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 /aRA0pL4 tLYjICkWnPDr+C8OuQd55G+StXVeZ/fYkCWdwMFdiKIoEoPV9imahTsqeuSx37ez+u0VydDCsNd245p6OB/a6EejTrgb0oqBzQfl3q7HcEabtKFGaKbG0mOVPjgtZgJX9tfajXBc0FRVp4EiyKtSVbYTqK+6cQTuXHo8q3azT+wVw+nz2lNAZN/bA0SbrvQIME/3CR25srXHM/ZnwrvAMO1r/8qd5412X43tHWulxXdphLl54RTWnVJcN2eN5a2RGjcQA59G6B4+J0X0ASNTkPiaVSA8d9leDZmdKuz2FyNOtebklJH6d1AKo6D9rzq0G6WQ26eA++Ot54B1f/HaqHyY/pjHNlBusE3zsGSaZIGPj4tuMGaVIuMcv57iDniSIxmz8sT/Z2tYbI0zMayYEkmpO9g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Jul 15, 2025 at 07:37:33AM -0700, Darrick J. Wong wrote: > On Tue, Jul 15, 2025 at 08:36:54AM -0400, Brian Foster wrote: > > On Mon, Jul 14, 2025 at 10:34:17PM -0700, Darrick J. Wong wrote: > > > On Mon, Jul 14, 2025 at 04:41:21PM -0400, Brian Foster wrote: > > > > iomap_zero_range() optimizes the partial eof block zeroing use case > > > > by force zeroing if the mapping is dirty. This is to avoid frequent > > > > flushing on file extending workloads, which hurts performance. > > > > > > > > Now that the folio batch mechanism provides a more generic solution > > > > and is used by the only real zero range user (XFS), this isolated > > > > optimization is no longer needed. Remove the unnecessary code and > > > > let callers use the folio batch or fall back to flushing by default. > > > > > > > > Signed-off-by: Brian Foster > > > > Reviewed-by: Christoph Hellwig > > > > > > Heh, I was staring at this last Friday chasing fuse+iomap bugs in > > > fallocate zerorange and straining to remember what this does. > > > Is this chunk still needed if the ->iomap_begin implementation doesn't > > > (or forgets to) grab the folio batch for iomap? > > > > > > > No, the hunk removed by this patch is just an optimization. The fallback > > code here flushes the range if it's dirty and retries the lookup (i.e. > > picking up unwritten conversions that were pending via dirty pagecache). > > That flush logic caused a performance regression in a particular > > workload, so this was introduced to mitigate that regression by just > > doing the zeroing for the first block or so if the folio is dirty. [1] > > > > The reason for removing it is more just for maintainability. XFS is > > really the only user here and it is changing over to the more generic > > batch mechanism, which effectively provides the same optimization, so > > this basically becomes dead/duplicate code. If an fs doesn't use the > > batch mechanism it will just fall back to the flush and retry approach, > > which can be slower but is functionally correct. > > Oh ok thanks for the reminder. > Reviewed-by: "Darrick J. Wong" > > > > My bug turned out to be a bug in my fuse+iomap design -- with the way > > > iomap_zero_range does things, you have to flush+unmap, punch the range > > > and zero the range. If you punch and realloc the range and *then* try > > > to zero the range, the new unwritten extents cause iomap to miss dirty > > > pages that fuse should've unmapped. Ooops. > > > > > > > I don't quite follow. How do you mean it misses dirty pages? > > Oops, I misspoke, the folios were clean. Let's say the pagecache is > sparsely populated with some folios for written space: > > -------fffff-------fffffff > wwwwwwwwwwwwwwwwwwwwwwwwww > > Now you tell it to go zero range the middle. fuse's fallocate code > issues the upcall to userspace, whch changes some mappings: > > -------fffff-------fffffff > wwwwwuuuuuuuuuuuwwwwwwwwww > > Only after the upcall returns does the kernel try to do the pagecache > zeroing. Unfortunately, the mapping changed to unwritten so > iomap_zero_range doesn't see the "fffff" and leaves its contents intact. > Ah, interesting. So presumably the fuse fs is not doing any cache managment, and this creates an unexpected inconsistency between pagecache and block state. So what's the solution to this for fuse+iomap? Invalidate the cache range before or after the callback or something? Brian > (Note: Non-iomap fuse defers everything to the fuse server so this isn't > a problem if the fuse server does all the zeroing itself.) > > --D > > > Brian > > > > [1] Details described in the commit log of fde4c4c3ec1c ("iomap: elide > > flush from partial eof zero range"). > > > > > --D > > > > > > > --- > > > > fs/iomap/buffered-io.c | 24 ------------------------ > > > > 1 file changed, 24 deletions(-) > > > > > > > > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c > > > > index 194e3cc0857f..d2bbed692c06 100644 > > > > --- a/fs/iomap/buffered-io.c > > > > +++ b/fs/iomap/buffered-io.c > > > > @@ -1484,33 +1484,9 @@ iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero, > > > > .private = private, > > > > }; > > > > struct address_space *mapping = inode->i_mapping; > > > > - unsigned int blocksize = i_blocksize(inode); > > > > - unsigned int off = pos & (blocksize - 1); > > > > - loff_t plen = min_t(loff_t, len, blocksize - off); > > > > int ret; > > > > bool range_dirty; > > > > > > > > - /* > > > > - * Zero range can skip mappings that are zero on disk so long as > > > > - * pagecache is clean. If pagecache was dirty prior to zero range, the > > > > - * mapping converts on writeback completion and so must be zeroed. > > > > - * > > > > - * The simplest way to deal with this across a range is to flush > > > > - * pagecache and process the updated mappings. To avoid excessive > > > > - * flushing on partial eof zeroing, special case it to zero the > > > > - * unaligned start portion if already dirty in pagecache. > > > > - */ > > > > - if (!iter.fbatch && off && > > > > - filemap_range_needs_writeback(mapping, pos, pos + plen - 1)) { > > > > - iter.len = plen; > > > > - while ((ret = iomap_iter(&iter, ops)) > 0) > > > > - iter.status = iomap_zero_iter(&iter, did_zero); > > > > - > > > > - iter.len = len - (iter.pos - pos); > > > > - if (ret || !iter.len) > > > > - return ret; > > > > - } > > > > - > > > > /* > > > > * To avoid an unconditional flush, check pagecache state and only flush > > > > * if dirty and the fs returns a mapping that might convert on > > > > -- > > > > 2.50.0 > > > > > > > > > > > > > > > >