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 B1CEE188A23 for ; Tue, 17 Sep 2024 18:31:43 +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=1726597903; cv=none; b=a3oMEk2biPoNFwUO3+Q8bsOa+Z7qzccF6mm8bpEcy3MseM5tjIb609hlyjGqja3FanMMZdrFCtLtTcYOqSQNHI6MURCMXJSxo795M9nFPIGxvGFykii3taa6Q293UOluJhqP6i5Fp8+kTv/Gs50ORx9EoolbZN64nNj8AUX5eqU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726597903; c=relaxed/simple; bh=OFkmp8k6kiWae3zcsCkN+xkWcQG5BEXjU0Z0X5ecOQ0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=gLv19Femse5TaA05xnz2wDAP2+QWwoOtYG0xD4+3j6L6ksWIUGZJ6V8AbBGBlHnDiySJhWf+Kr5+3B8+gpXcTFgyXgbw3bWvsCJ8ckrUGvqb05GCxbkxy0/DxX9tHPeIbCrgSvOb7TVatrkjrTnYN4ya4IhrzJTRmxBXzlckjjc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=BCZrj9mD; 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="BCZrj9mD" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 43247C4CEC5; Tue, 17 Sep 2024 18:31:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1726597903; bh=OFkmp8k6kiWae3zcsCkN+xkWcQG5BEXjU0Z0X5ecOQ0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=BCZrj9mDAH9hcEgE4wwSek1ZGeah3pCcoPLoDmv8d4P4rzfqD3HXhd284/yA0CAaA gCKnzVbBTcLtf4LPW08LLUaLmSBlwrY6r6gahV39+TQQESjXMxpdsVlYKdT7hFrkIB Yl3WuAVQ4VeXLaXZZ3JtO9hKwGWuK6vFDe1uDV8g8ALRkCGz5+X1E3BaZYjbGYvelK UZAsZMEpRhgenSEN8JV0oZz9aKOMG0t/fBOR68shbUTlERkCE31hGOgZgsoL/hs6k9 qp83UgHY4CEaM4GbMX0u5yJ481UXAJfNXoUM5DqGiVaqJnyEE5/v33fCjET6och5Fs MUmxnlHwfI1YQ== Date: Tue, 17 Sep 2024 11:31:42 -0700 From: "Darrick J. Wong" To: Brian Foster Cc: linux-xfs@vger.kernel.org Subject: Re: [PATCH 2/1] xfs: don't free cowblocks from under dirty pagecache on unshare Message-ID: <20240917183142.GI182194@frogsfrogsfrogs> References: <20240903124713.23289-1-bfoster@redhat.com> <20240906114051.120743-1-bfoster@redhat.com> Precedence: bulk X-Mailing-List: linux-xfs@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: <20240906114051.120743-1-bfoster@redhat.com> On Fri, Sep 06, 2024 at 07:40:51AM -0400, Brian Foster wrote: > fallocate unshare mode explicitly breaks extent sharing. When a > command completes, it checks the data fork for any remaining shared > extents to determine whether the reflink inode flag and COW fork > preallocation can be removed. This logic doesn't consider in-core > pagecache and I/O state, however, which means we can unsafely remove > COW fork blocks that are still needed under certain conditions. > > For example, consider the following command sequence: > > xfs_io -fc "pwrite 0 1k" -c "reflink 0 256k 1k" \ > -c "pwrite 0 32k" -c "funshare 0 1k" > > This allocates a data block at offset 0, shares it, and then > overwrites it with a larger buffered write. The overwrite triggers > COW fork preallocation, 32 blocks by default, which maps the entire > 32k write to delalloc in the COW fork. All but the shared block at > offset 0 remains hole mapped in the data fork. The unshare command > redirties and flushes the folio at offset 0, removing the only > shared extent from the inode. Since the inode no longer maps shared > extents, unshare purges the COW fork before the remaining 28k may > have written back. > > This leaves dirty pagecache backed by holes, which writeback quietly > skips, thus leaving clean, non-zeroed pagecache over holes in the > file. To verify, fiemap shows holes in the first 32k of the file and > reads return different data across a remount: > > $ xfs_io -c "fiemap -v" > : > EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS > ... > 1: [8..511]: hole 504 > ... > $ xfs_io -c "pread -v 4k 8" > 00001000: cd cd cd cd cd cd cd cd ........ > $ umount ; mount > $ xfs_io -c "pread -v 4k 8" > 00001000: 00 00 00 00 00 00 00 00 ........ > > To avoid this problem, make unshare follow the same rules used for > background cowblock scanning and never purge the COW fork for inodes > with dirty pagecache or in-flight I/O. > > Fixes: 46afb0628b ("xfs: only flush the unshared range in xfs_reflink_unshare") > Signed-off-by: Brian Foster Question: Does xfs_repair report orphaned cow staging blocks after this? There's a longstanding bug that I've seen in the long soak xfs/286 VM where we slowly leak cow fork blocks (~80 per ~1 billion fsxops over 7 days). Anyhow this looks correct on its own so Reviewed-by: Darrick J. Wong --D > --- > > Here's another COW issue I came across via some unshare testing. A quick > hack to enable unshare in fsx uncovered it. I'll follow up with a proper > patch for that. > > I'm sending this as a 2/1 here just to reflect patch order in my local > tree. Also note that I haven't explicitly tested the fixes commit, but a > quick test to switch back to the old full flush behavior on latest > master also makes the problem go away, so I suspect that's where the > regression was introduced. > > Brian > > fs/xfs/xfs_icache.c | 8 +------- > fs/xfs/xfs_reflink.c | 3 +++ > fs/xfs/xfs_reflink.h | 19 +++++++++++++++++++ > 3 files changed, 23 insertions(+), 7 deletions(-) > > diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c > index 900a6277d931..a1b34e6ccfe2 100644 > --- a/fs/xfs/xfs_icache.c > +++ b/fs/xfs/xfs_icache.c > @@ -1278,13 +1278,7 @@ xfs_prep_free_cowblocks( > */ > if (!sync && inode_is_open_for_write(VFS_I(ip))) > return false; > - if ((VFS_I(ip)->i_state & I_DIRTY_PAGES) || > - mapping_tagged(VFS_I(ip)->i_mapping, PAGECACHE_TAG_DIRTY) || > - mapping_tagged(VFS_I(ip)->i_mapping, PAGECACHE_TAG_WRITEBACK) || > - atomic_read(&VFS_I(ip)->i_dio_count)) > - return false; > - > - return true; > + return xfs_can_free_cowblocks(ip); > } > > /* > diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c > index 6fde6ec8092f..5bf6682e701b 100644 > --- a/fs/xfs/xfs_reflink.c > +++ b/fs/xfs/xfs_reflink.c > @@ -1595,6 +1595,9 @@ xfs_reflink_clear_inode_flag( > > ASSERT(xfs_is_reflink_inode(ip)); > > + if (!xfs_can_free_cowblocks(ip)) > + return 0; > + > error = xfs_reflink_inode_has_shared_extents(*tpp, ip, &needs_flag); > if (error || needs_flag) > return error; > diff --git a/fs/xfs/xfs_reflink.h b/fs/xfs/xfs_reflink.h > index fb55e4ce49fa..4a58e4533671 100644 > --- a/fs/xfs/xfs_reflink.h > +++ b/fs/xfs/xfs_reflink.h > @@ -6,6 +6,25 @@ > #ifndef __XFS_REFLINK_H > #define __XFS_REFLINK_H 1 > > +/* > + * Check whether it is safe to free COW fork blocks from an inode. It is unsafe > + * to do so when an inode has dirty cache or I/O in-flight, even if no shared > + * extents exist in the data fork, because outstanding I/O may target blocks > + * that were speculatively allocated to the COW fork. > + */ > +static inline bool > +xfs_can_free_cowblocks(struct xfs_inode *ip) > +{ > + struct inode *inode = VFS_I(ip); > + > + if ((inode->i_state & I_DIRTY_PAGES) || > + mapping_tagged(inode->i_mapping, PAGECACHE_TAG_DIRTY) || > + mapping_tagged(inode->i_mapping, PAGECACHE_TAG_WRITEBACK) || > + atomic_read(&inode->i_dio_count)) > + return false; > + return true; > +} > + > extern int xfs_reflink_trim_around_shared(struct xfs_inode *ip, > struct xfs_bmbt_irec *irec, bool *shared); > int xfs_bmap_trim_cow(struct xfs_inode *ip, struct xfs_bmbt_irec *imap, > -- > 2.45.0 > >