From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 C185884A3E for ; Tue, 15 Jul 2025 16:16:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752596203; cv=none; b=a9btDhV0wQu+OfD+trffJBNY7rxHWWse2Qez692o6z8svbrciyI2ATrBweD0s8c8F0Rs/FULTYKTPgnflo/1X1DYkVdfGJ6Qv26pXAoNzLPVOW03j58zbej7UDuSVhT7O5iYwqZ8gAC+Pgej79HX/bW/CjTeeLjHCZo5Lbi7nMs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752596203; c=relaxed/simple; bh=8cXGOXH0NmsGA9LMeMB3nYmE7IQt/S/JV4bLliS9PAE=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=tX6h0WKQd0GYlVM4yrsLv5QYQbgS1cWi1qYSQeIqN4DeetHp7JHWX40A2BQLR7sB+h3Rq/A9D8Cc4KEVQWdXRDQjNSfrc/X/5MQnE0ePOFc+1zccQV7U1SiizYNAqF/bEs4yxip8jc04O8g5BAW0T6rqeh2OboRk9myC6G4cums= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=Pzxj/D6e; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Pzxj/D6e" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1752596200; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=PjzkI9qth/2TE7efHICxEZZlRM5MU26dY9ECAyolYoQ=; b=Pzxj/D6eKbTJZELtHpjct6tFYitoPLm3WZ1ufjQuiiiTmSQaDoxPxIMa0IZlpt1cRJOEGB ecSk2VP30+jeuhjn6mv6MFA1tuEQu+rUKc0CWFGGmrAhD3TOYrihV2pJD0yt6Gk5FRxOdd h8BzHPolRqVS3trwsYx/rrbfvETaDmo= Received: from mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-568-ZjihElmHNuubUzssVe7u3A-1; Tue, 15 Jul 2025 12:16:36 -0400 X-MC-Unique: ZjihElmHNuubUzssVe7u3A-1 X-Mimecast-MFC-AGG-ID: ZjihElmHNuubUzssVe7u3A_1752596195 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 0DCDD19560B7; Tue, 15 Jul 2025 16:16:35 +0000 (UTC) Received: from bfoster (unknown [10.22.64.43]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 34259180035C; Tue, 15 Jul 2025 16:16:33 +0000 (UTC) Date: Tue, 15 Jul 2025 12:20:14 -0400 From: Brian Foster To: "Darrick J. 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> 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: <20250715143733.GO2672029@frogsfrogsfrogs> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 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 > > > > > > > > > > > > > > > >