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 9746B19F464; Tue, 15 Jul 2025 16:30:58 +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=1752597058; cv=none; b=V0ICyWZOOzaD/GUVkd3pnaln4jn8s88PyYaLOJ05vC4dljwGMp4g+nbBrrsaoLNwf0cEvarxc47eNUcLRSCPZVvukLSbGK+ZOQQ6vAcGsexLDBbjC3XYXl1cPwg2DJp5XtNBoXk1gUXsdTdNRoUAk0JJgr4n8BELmpPNAjuP3vo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752597058; c=relaxed/simple; bh=boqYpnyjQkrnOLC5/e/LhE8fcrdUQRoDXg4rbAi8Tic=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=NQWAB64qpm5g365dXbyN3L1ama/Bw+Eadg5UqZRTu0uBREDAqsF/s30cs1ZXRiyEOxifhHKTshMxiOkKX1iXbGy0D3G8gbjnzuwUFeVJIurqEPTTN+ZZojGTw9ZGsSiKtszGfKMLQDvDT0guTJs0EAMogtXXRIF8sQFhPTuqihE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=RhayR7Dp; 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="RhayR7Dp" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2A3C3C4CEE3; Tue, 15 Jul 2025 16:30:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1752597058; bh=boqYpnyjQkrnOLC5/e/LhE8fcrdUQRoDXg4rbAi8Tic=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=RhayR7DpK5GaiEctLq2Vpx92vMICy3+cmIoQXsIsYM98vRGeJvQMBEcoOK27BinYU CLHx2zmUQvUJew1UstlfkOO53PfpcTXIbeqAl++QnD/YyHZS9aSWAjQrXm33D37kab LneWhUTyHxRREgXPuFoSzaQKV9u9zUH4wRJXaQyUUaupM3yt9tRYJQ1ofwFAb2Hz4u CCACbT4g9Z/6chKnbP0WJxAWBnLdvRaP78nJOBFYPhcVzQAMwvM4KfALuceTFp3ECA d9IvQOGQhcwXOfwppkluzwsQ7l/nqR6PApSn/ODqqNNSF5Zt30zejsKr2mbgapk2uY Gv1NLAxRcAR5g== Date: Tue, 15 Jul 2025 09:30:57 -0700 From: "Darrick J. Wong" To: Brian Foster 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: <20250715163057.GC2672049@frogsfrogsfrogs> References: <20250714204122.349582-1-bfoster@redhat.com> <20250714204122.349582-7-bfoster@redhat.com> <20250715053417.GR2672049@frogsfrogsfrogs> <20250715143733.GO2672029@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: On Tue, Jul 15, 2025 at 12:20:14PM -0400, Brian Foster wrote: > 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? Port xfs_flush_unmap_range, I think. --D > 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 > > > > > > > > > > > > > > > > > > > > > > > >