Re: [PATCH 8/8] xfs: introduce an always_cow mode

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, Feb 18, 2019 at 09:25:45PM -0800, Darrick J. Wong wrote:
> On Mon, Feb 18, 2019 at 10:18:27AM +0100, Christoph Hellwig wrote:
> > Add a mode where XFS never overwrites existing blocks in place.  This
> > is to aid debugging our COW code, and also put infatructure in place
> > for things like possible future support for zoned block devices, which
> > can't support overwrites.
> > 
> > This mode is enabled globally by doing a:
> > 
> >     echo 1 > /sys/fs/xfs/debug/always_cow
> > 
> > Note that the parameter is global to allow running all tests in xfstests
> > easily in this mode, which would not easily be possible with a per-fs
> > sysfs file.
> > 
> > In always_cow mode persistent preallocations are disabled, and fallocate
> > will fail when called with a 0 mode (with our without
> > FALLOC_FL_KEEP_SIZE), and not create unwritten extent for zeroed space
> > when called with FALLOC_FL_ZERO_RANGE or FALLOC_FL_UNSHARE_RANGE.
> > 
> > There are a few interesting xfstests failures when run in always_cow
> > mode:
> > 
> >  - generic/392 fails because the bytes used in the file used to test
> >    hole punch recovery are less after the log replay.  This is
> >    because the blocks written and then punched out are only freed
> >    with a delay due to the logging mechanism.
> >  - xfs/170 will fail as the already fragile file streams mechanism
> >    doesn't seem to interact well with the COW allocator
> >  - xfs/180 xfs/182 xfs/192 xfs/198 xfs/204 and xfs/208 will claim
> >    the file system is badly fragmented, but there is not much we
> >    can do to avoid that when always writing out of place
> >  - xfs/205 fails because overwriting a file in always_cow mode
> >    will require new space allocation and the assumption in the
> >    test thus don't work anymore.
> >  - xfs/326 fails to modify the file at all in always_cow mode after
> >    injecting the refcount error, leading to an unexpected md5sum
> >    after the remount, but that again is expected
> 
> Yeah, I saw failures in a bunch of tests when running always_cow against
> a 4k blocksize rmap/reflink filesystem:

/me wakes up and looks...

> xfs/056
> xfs/064
> xfs/060
> xfs/066
> xfs/068
> xfs/283
> xfs/296

xfsrestore tries to RESVSP space for its index file and fails,
presumably because the vfs intercepts the RESVSP ioctl and calls
vfs_fallocate.  But then xfsrestore seems to run ALLOCSP, which afaict
succeeds... ?

Two questions:

- Are you going to mask off xfs_ioc_space from alwayscow inodes like you
  did for xfs_file_fallocate?

- What are we going to do for xfsrestore if it encounters a filesystem
  where everything is alwayscow?

> xfs/205

"disk full, expected" ?

/me wonders, since this is an ENOSPC test, why would it fail if we run
out of space.

> xfs/192
> xfs/198

whines about fragmentation as noted; when we start working on a more
user-visible alwayscow mode we're going to have to figure out a way to
notrun these or something...

> generic/392

There's a little more going on here than what you said above -- the
failing test is a 1k punch, which just writes zeroes to the page cache,
thereby invoking the COW mechanism.  We create a 256k speculative
preallocation in the COW fork, which is reported through i_delayed_blks.
After shutting down and remounting that speculative preallocation is no
longer attached to the inode, so the block count is lower.

Anyway, the test needs some kind of adjustment to work around this,
though for some reason setting cowextsize=1fsb to disable the
speculative cow preallocations still results in a discrepancy of 1 fsb.

> generic/476

lockdep warning about reclaim during fs freeze, not related...

> xfs/1501

unreleased test, ignore this too...

> xfs/170

filestreams, who knows...

> xfs/450

Missing _require_xfs_io_command "falloc", I'll send a patch.

> generic/475

unrelated quota leak problems, yay...

> 
> Will have a look at those tomorrow...

And now back to reviewing patches.

--D

> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > ---
> >  fs/xfs/xfs_aops.c      |  2 +-
> >  fs/xfs/xfs_bmap_util.c |  9 +++------
> >  fs/xfs/xfs_file.c      | 27 ++++++++++++++++++++-------
> >  fs/xfs/xfs_iomap.c     | 28 ++++++++++++++++++----------
> >  fs/xfs/xfs_mount.h     |  1 +
> >  fs/xfs/xfs_reflink.c   | 28 ++++++++++++++++++++++++----
> >  fs/xfs/xfs_reflink.h   | 13 +++++++++++++
> >  fs/xfs/xfs_super.c     | 15 +++++++++++----
> >  fs/xfs/xfs_sysctl.h    |  1 +
> >  fs/xfs/xfs_sysfs.c     | 24 ++++++++++++++++++++++++
> >  10 files changed, 116 insertions(+), 32 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> > index 983d11c27d32..7b8bb6bde981 100644
> > --- a/fs/xfs/xfs_aops.c
> > +++ b/fs/xfs/xfs_aops.c
> > @@ -1023,7 +1023,7 @@ xfs_vm_bmap(
> >  	 * Since we don't pass back blockdev info, we can't return bmap
> >  	 * information for rt files either.
> >  	 */
> > -	if (xfs_is_reflink_inode(ip) || XFS_IS_REALTIME_INODE(ip))
> > +	if (xfs_is_cow_inode(ip) || XFS_IS_REALTIME_INODE(ip))
> >  		return 0;
> >  	return iomap_bmap(mapping, block, &xfs_iomap_ops);
> >  }
> > diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> > index 1ee8c5539fa4..2db43ff4f8b5 100644
> > --- a/fs/xfs/xfs_bmap_util.c
> > +++ b/fs/xfs/xfs_bmap_util.c
> > @@ -1162,16 +1162,13 @@ xfs_zero_file_space(
> >  	 * by virtue of the hole punch.
> >  	 */
> >  	error = xfs_free_file_space(ip, offset, len);
> > -	if (error)
> > -		goto out;
> > +	if (error || xfs_is_always_cow_inode(ip))
> > +		return error;
> >  
> > -	error = xfs_alloc_file_space(ip, round_down(offset, blksize),
> > +	return xfs_alloc_file_space(ip, round_down(offset, blksize),
> >  				     round_up(offset + len, blksize) -
> >  				     round_down(offset, blksize),
> >  				     XFS_BMAPI_PREALLOC);
> > -out:
> > -	return error;
> > -
> >  }
> >  
> >  static int
> > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > index 1d07dcfbbff3..770cc2edf777 100644
> > --- a/fs/xfs/xfs_file.c
> > +++ b/fs/xfs/xfs_file.c
> > @@ -507,7 +507,7 @@ xfs_file_dio_aio_write(
> >  		 * We can't properly handle unaligned direct I/O to reflink
> >  		 * files yet, as we can't unshare a partial block.
> >  		 */
> > -		if (xfs_is_reflink_inode(ip)) {
> > +		if (xfs_is_cow_inode(ip)) {
> >  			trace_xfs_reflink_bounce_dio_write(ip, iocb->ki_pos, count);
> >  			return -EREMCHG;
> >  		}
> > @@ -872,14 +872,27 @@ xfs_file_fallocate(
> >  				goto out_unlock;
> >  		}
> >  
> > -		if (mode & FALLOC_FL_ZERO_RANGE)
> > +		if (mode & FALLOC_FL_ZERO_RANGE) {
> >  			error = xfs_zero_file_space(ip, offset, len);
> > -		else {
> > -			if (mode & FALLOC_FL_UNSHARE_RANGE) {
> > -				error = xfs_reflink_unshare(ip, offset, len);
> > -				if (error)
> > -					goto out_unlock;
> > +		} else if (mode & FALLOC_FL_UNSHARE_RANGE) {
> > +			error = xfs_reflink_unshare(ip, offset, len);
> > +			if (error)
> > +				goto out_unlock;
> > +
> > +			if (!xfs_is_always_cow_inode(ip)) {
> > +				error = xfs_alloc_file_space(ip, offset, len,
> > +						XFS_BMAPI_PREALLOC);
> >  			}
> > +		} else {
> > +			/*
> > +			 * If always_cow mode we can't use preallocations and
> > +			 * thus should not create them.
> > +			 */
> > +			if (xfs_is_always_cow_inode(ip)) {
> > +				error = -EOPNOTSUPP;
> > +				goto out_unlock;
> > +			}
> > +
> >  			error = xfs_alloc_file_space(ip, offset, len,
> >  						     XFS_BMAPI_PREALLOC);
> >  		}
> > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> > index 7b71dcc97ef0..72533e9dddc6 100644
> > --- a/fs/xfs/xfs_iomap.c
> > +++ b/fs/xfs/xfs_iomap.c
> > @@ -395,12 +395,13 @@ xfs_quota_calc_throttle(
> >  STATIC xfs_fsblock_t
> >  xfs_iomap_prealloc_size(
> >  	struct xfs_inode	*ip,
> > +	int			whichfork,
> >  	loff_t			offset,
> >  	loff_t			count,
> >  	struct xfs_iext_cursor	*icur)
> >  {
> >  	struct xfs_mount	*mp = ip->i_mount;
> > -	struct xfs_ifork	*ifp = XFS_IFORK_PTR(ip, XFS_DATA_FORK);
> > +	struct xfs_ifork	*ifp = XFS_IFORK_PTR(ip, whichfork);
> >  	xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
> >  	struct xfs_bmbt_irec	prev;
> >  	int			shift = 0;
> > @@ -593,7 +594,11 @@ xfs_file_iomap_begin_delay(
> >  	 * themselves.  Second the lookup in the extent list is generally faster
> >  	 * than going out to the shared extent tree.
> >  	 */
> > -	if (xfs_is_reflink_inode(ip)) {
> > +	if (xfs_is_cow_inode(ip)) {
> > +		if (!ip->i_cowfp) {
> > +			ASSERT(!xfs_is_reflink_inode(ip));
> > +			xfs_ifork_init_cow(ip);
> > +		}
> >  		cow_eof = !xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb,
> >  				&ccur, &cmap);
> >  		if (!cow_eof && cmap.br_startoff <= offset_fsb) {
> > @@ -609,7 +614,7 @@ xfs_file_iomap_begin_delay(
> >  		 * overwriting shared extents.   This includes zeroing of
> >  		 * existing extents that contain data.
> >  		 */
> > -		if (!xfs_is_reflink_inode(ip) ||
> > +		if (!xfs_is_cow_inode(ip) ||
> >  		    ((flags & IOMAP_ZERO) && imap.br_state != XFS_EXT_NORM)) {
> >  			trace_xfs_iomap_found(ip, offset, count, XFS_DATA_FORK,
> >  					&imap);
> > @@ -619,7 +624,7 @@ xfs_file_iomap_begin_delay(
> >  		xfs_trim_extent(&imap, offset_fsb, end_fsb - offset_fsb);
> >  
> >  		/* Trim the mapping to the nearest shared extent boundary. */
> > -		error = xfs_reflink_trim_around_shared(ip, &imap, &shared);
> > +		error = xfs_inode_need_cow(ip, &imap, &shared);
> >  		if (error)
> >  			goto out_unlock;
> >  
> > @@ -648,15 +653,18 @@ xfs_file_iomap_begin_delay(
> >  		 */
> >  		count = min_t(loff_t, count, 1024 * PAGE_SIZE);
> >  		end_fsb = min(XFS_B_TO_FSB(mp, offset + count), maxbytes_fsb);
> > +
> > +		if (xfs_is_always_cow_inode(ip))
> > +			whichfork = XFS_COW_FORK;
> >  	}
> >  
> >  	error = xfs_qm_dqattach_locked(ip, false);
> >  	if (error)
> >  		goto out_unlock;
> >  
> > -	if (eof && whichfork == XFS_DATA_FORK) {
> > -		prealloc_blocks = xfs_iomap_prealloc_size(ip, offset, count,
> > -				&icur);
> > +	if (eof) {
> > +		prealloc_blocks = xfs_iomap_prealloc_size(ip, whichfork, offset,
> > +				count, &icur);
> >  		if (prealloc_blocks) {
> >  			xfs_extlen_t	align;
> >  			xfs_off_t	end_offset;
> > @@ -867,7 +875,7 @@ xfs_ilock_for_iomap(
> >  	 * COW writes may allocate delalloc space or convert unwritten COW
> >  	 * extents, so we need to make sure to take the lock exclusively here.
> >  	 */
> > -	if (xfs_is_reflink_inode(ip) && is_write) {
> > +	if (xfs_is_cow_inode(ip) && is_write) {
> >  		/*
> >  		 * FIXME: It could still overwrite on unshared extents and not
> >  		 * need allocation.
> > @@ -901,7 +909,7 @@ xfs_ilock_for_iomap(
> >  	 * check, so if we got ILOCK_SHARED for a write and but we're now a
> >  	 * reflink inode we have to switch to ILOCK_EXCL and relock.
> >  	 */
> > -	if (mode == XFS_ILOCK_SHARED && is_write && xfs_is_reflink_inode(ip)) {
> > +	if (mode == XFS_ILOCK_SHARED && is_write && xfs_is_cow_inode(ip)) {
> >  		xfs_iunlock(ip, mode);
> >  		mode = XFS_ILOCK_EXCL;
> >  		goto relock;
> > @@ -973,7 +981,7 @@ xfs_file_iomap_begin(
> >  	 * Break shared extents if necessary. Checks for non-blocking IO have
> >  	 * been done up front, so we don't need to do them here.
> >  	 */
> > -	if (xfs_is_reflink_inode(ip)) {
> > +	if (xfs_is_cow_inode(ip)) {
> >  		struct xfs_bmbt_irec	orig = imap;
> >  
> >  		/* if zeroing doesn't need COW allocation, then we are done. */
> > diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
> > index a33f45077867..fa8b37d61838 100644
> > --- a/fs/xfs/xfs_mount.h
> > +++ b/fs/xfs/xfs_mount.h
> > @@ -194,6 +194,7 @@ typedef struct xfs_mount {
> >  	 */
> >  	uint32_t		m_generation;
> >  
> > +	bool			m_always_cow;
> >  	bool			m_fail_unmount;
> >  #ifdef DEBUG
> >  	/*
> > diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> > index f84b37fa4f17..e2d9179bd50d 100644
> > --- a/fs/xfs/xfs_reflink.c
> > +++ b/fs/xfs/xfs_reflink.c
> > @@ -192,7 +192,7 @@ xfs_reflink_trim_around_shared(
> >  	int			error = 0;
> >  
> >  	/* Holes, unwritten, and delalloc extents cannot be shared */
> > -	if (!xfs_is_reflink_inode(ip) || !xfs_bmap_is_real_extent(irec)) {
> > +	if (!xfs_is_cow_inode(ip) || !xfs_bmap_is_real_extent(irec)) {
> >  		*shared = false;
> >  		return 0;
> >  	}
> > @@ -234,6 +234,23 @@ xfs_reflink_trim_around_shared(
> >  	}
> >  }
> >  
> > +bool
> > +xfs_inode_need_cow(
> 
> xfs_inode_trim_to_cow() ?
> 
> Otherwise generally looks ok to me, but it's late so I'll send this now
> and get back to review in the morning.
> 
> --D
> 
> > +	struct xfs_inode	*ip,
> > +	struct xfs_bmbt_irec	*imap,
> > +	bool			*shared)
> > +{
> > +	/* We can't update any real extents in always COW mode. */
> > +	if (xfs_is_always_cow_inode(ip) &&
> > +	    !isnullstartblock(imap->br_startblock)) {
> > +		*shared = true;
> > +		return 0;
> > +	}
> > +
> > +	/* Trim the mapping to the nearest shared extent boundary. */
> > +	return xfs_reflink_trim_around_shared(ip, imap, shared);
> > +}
> > +
> >  static int
> >  xfs_reflink_convert_cow_locked(
> >  	struct xfs_inode	*ip,
> > @@ -321,7 +338,7 @@ xfs_find_trim_cow_extent(
> >  	if (got.br_startoff > offset_fsb) {
> >  		xfs_trim_extent(imap, imap->br_startoff,
> >  				got.br_startoff - imap->br_startoff);
> > -		return xfs_reflink_trim_around_shared(ip, imap, shared);
> > +		return xfs_inode_need_cow(ip, imap, shared);
> >  	}
> >  
> >  	*shared = true;
> > @@ -356,7 +373,10 @@ xfs_reflink_allocate_cow(
> >  	xfs_extlen_t		resblks = 0;
> >  
> >  	ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
> > -	ASSERT(xfs_is_reflink_inode(ip));
> > +	if (!ip->i_cowfp) {
> > +		ASSERT(!xfs_is_reflink_inode(ip));
> > +		xfs_ifork_init_cow(ip);
> > +	}
> >  
> >  	error = xfs_find_trim_cow_extent(ip, imap, shared, &found);
> >  	if (error || !*shared)
> > @@ -542,7 +562,7 @@ xfs_reflink_cancel_cow_range(
> >  	int			error;
> >  
> >  	trace_xfs_reflink_cancel_cow_range(ip, offset, count);
> > -	ASSERT(xfs_is_reflink_inode(ip));
> > +	ASSERT(ip->i_cowfp);
> >  
> >  	offset_fsb = XFS_B_TO_FSBT(ip->i_mount, offset);
> >  	if (count == NULLFILEOFF)
> > diff --git a/fs/xfs/xfs_reflink.h b/fs/xfs/xfs_reflink.h
> > index 4a9e3cd4768a..2a3052fbe23e 100644
> > --- a/fs/xfs/xfs_reflink.h
> > +++ b/fs/xfs/xfs_reflink.h
> > @@ -6,11 +6,24 @@
> >  #ifndef __XFS_REFLINK_H
> >  #define __XFS_REFLINK_H 1
> >  
> > +static inline bool xfs_is_always_cow_inode(struct xfs_inode *ip)
> > +{
> > +	return ip->i_mount->m_always_cow &&
> > +		xfs_sb_version_hasreflink(&ip->i_mount->m_sb);
> > +}
> > +
> > +static inline bool xfs_is_cow_inode(struct xfs_inode *ip)
> > +{
> > +	return xfs_is_reflink_inode(ip) || xfs_is_always_cow_inode(ip);
> > +}
> > +
> >  extern int xfs_reflink_find_shared(struct xfs_mount *mp, struct xfs_trans *tp,
> >  		xfs_agnumber_t agno, xfs_agblock_t agbno, xfs_extlen_t aglen,
> >  		xfs_agblock_t *fbno, xfs_extlen_t *flen, bool find_maximal);
> >  extern int xfs_reflink_trim_around_shared(struct xfs_inode *ip,
> >  		struct xfs_bmbt_irec *irec, bool *shared);
> > +bool xfs_inode_need_cow(struct xfs_inode *ip, struct xfs_bmbt_irec *imap,
> > +		bool *shared);
> >  
> >  extern int xfs_reflink_allocate_cow(struct xfs_inode *ip,
> >  		struct xfs_bmbt_irec *imap, bool *shared, uint *lockmode,
> > diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> > index c9097cb0b955..6be183a391b6 100644
> > --- a/fs/xfs/xfs_super.c
> > +++ b/fs/xfs/xfs_super.c
> > @@ -1729,11 +1729,18 @@ xfs_fs_fill_super(
> >  		}
> >  	}
> >  
> > -	if (xfs_sb_version_hasreflink(&mp->m_sb) && mp->m_sb.sb_rblocks) {
> > -		xfs_alert(mp,
> > +	if (xfs_sb_version_hasreflink(&mp->m_sb)) {
> > +		if (mp->m_sb.sb_rblocks) {
> > +			xfs_alert(mp,
> >  	"reflink not compatible with realtime device!");
> > -		error = -EINVAL;
> > -		goto out_filestream_unmount;
> > +			error = -EINVAL;
> > +			goto out_filestream_unmount;
> > +		}
> > +
> > +		if (xfs_globals.always_cow) {
> > +			xfs_info(mp, "using DEBUG-only always_cow mode.");
> > +			mp->m_always_cow = true;
> > +		}
> >  	}
> >  
> >  	if (xfs_sb_version_hasrmapbt(&mp->m_sb) && mp->m_sb.sb_rblocks) {
> > diff --git a/fs/xfs/xfs_sysctl.h b/fs/xfs/xfs_sysctl.h
> > index 168488130a19..ad7f9be13087 100644
> > --- a/fs/xfs/xfs_sysctl.h
> > +++ b/fs/xfs/xfs_sysctl.h
> > @@ -85,6 +85,7 @@ struct xfs_globals {
> >  	int	log_recovery_delay;	/* log recovery delay (secs) */
> >  	int	mount_delay;		/* mount setup delay (secs) */
> >  	bool	bug_on_assert;		/* BUG() the kernel on assert failure */
> > +	bool	always_cow;		/* use COW fork for all overwrites */
> >  };
> >  extern struct xfs_globals	xfs_globals;
> >  
> > diff --git a/fs/xfs/xfs_sysfs.c b/fs/xfs/xfs_sysfs.c
> > index cd6a994a7250..cabda13f3c64 100644
> > --- a/fs/xfs/xfs_sysfs.c
> > +++ b/fs/xfs/xfs_sysfs.c
> > @@ -183,10 +183,34 @@ mount_delay_show(
> >  }
> >  XFS_SYSFS_ATTR_RW(mount_delay);
> >  
> > +static ssize_t
> > +always_cow_store(
> > +	struct kobject	*kobject,
> > +	const char	*buf,
> > +	size_t		count)
> > +{
> > +	ssize_t		ret;
> > +
> > +	ret = kstrtobool(buf, &xfs_globals.always_cow);
> > +	if (ret < 0)
> > +		return ret;
> > +	return count;
> > +}
> > +
> > +static ssize_t
> > +always_cow_show(
> > +	struct kobject	*kobject,
> > +	char		*buf)
> > +{
> > +	return snprintf(buf, PAGE_SIZE, "%d\n", xfs_globals.always_cow);
> > +}
> > +XFS_SYSFS_ATTR_RW(always_cow);
> > +
> >  static struct attribute *xfs_dbg_attrs[] = {
> >  	ATTR_LIST(bug_on_assert),
> >  	ATTR_LIST(log_recovery_delay),
> >  	ATTR_LIST(mount_delay),
> > +	ATTR_LIST(always_cow),
> >  	NULL,
> >  };
> >  
> > -- 
> > 2.20.1
> >