fix FITRIM with non-discard capable RT device v2

fix FITRIM with non-discard capable RT device v2

[Christoph Hellwig]

Hi all,

this series fixes FITRIM when an RT device that does not support
discards is present.

Changes since v1:
 - explicitly reject the discard capable RT device and non-discard capable
   main device case because it is too much of a mess to handle and not
   practically useful

Diffstat:
 xfs_discard.c |   57 +++++++++++++++++++++++++++++++--------------------------
 1 file changed, 31 insertions(+), 26 deletions(-)

[PATCH 1/2] xfs: remove a stale comment in xfs_ioc_trim

[Christoph Hellwig]

There is no truncating down going on here, the code has changed multiple
times since the comment was added with the initial FITRIM implementation
and it doesn't make sense in the current context.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/xfs/xfs_discard.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/fs/xfs/xfs_discard.c b/fs/xfs/xfs_discard.c
index 80586336276c19..d56efe9eae2cef 100644
--- a/fs/xfs/xfs_discard.c
+++ b/fs/xfs/xfs_discard.c
@@ -720,13 +720,6 @@ xfs_ioc_trim(
 	range.minlen = max_t(u64, granularity, range.minlen);
 	minlen = XFS_B_TO_FSB(mp, range.minlen);
 
-	/*
-	 * Truncating down the len isn't actually quite correct, but using
-	 * BBTOB would mean we trivially get overflows for values
-	 * of ULLONG_MAX or slightly lower.  And ULLONG_MAX is the default
-	 * used by the fstrim application.  In the end it really doesn't
-	 * matter as trimming blocks is an advisory interface.
-	 */
 	max_blocks = mp->m_sb.sb_dblocks + mp->m_sb.sb_rblocks;
 	if (range.start >= XFS_FSB_TO_B(mp, max_blocks) ||
 	    range.minlen > XFS_FSB_TO_B(mp, mp->m_ag_max_usable) ||
-- 
2.43.0

[PATCH 2/2] xfs: don't extend the FITRIM range if the rt device does not support discard

[Christoph Hellwig]

Fix the newly added discard support to only offer a FITRIM range that
spans the RT device in addition to the main device if the RT device
actually supports discard.  Without this we'll incorrectly accept
a larger range than actually supported and confuse user space if the
RT device does not support discard.  This can easily happen when the
main device is a SSD but the RT device is a hard driver.

Move the code around a bit to keep the max_blocks and granularity
assignments together and explicitly reject that case where only the
RT device supports discard, as that does not fit the way the FITRIM ABI
works very well and is a very fringe use case.

Fixes: 3ba3ab1f6719 ("xfs: enable FITRIM on the realtime device")
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 fs/xfs/xfs_discard.c | 50 +++++++++++++++++++++++++++-----------------
 1 file changed, 31 insertions(+), 19 deletions(-)

diff --git a/fs/xfs/xfs_discard.c b/fs/xfs/xfs_discard.c
index d56efe9eae2cef..77d9d2b39f9b00 100644
--- a/fs/xfs/xfs_discard.c
+++ b/fs/xfs/xfs_discard.c
@@ -685,27 +685,18 @@ xfs_ioc_trim(
 	struct xfs_mount		*mp,
 	struct fstrim_range __user	*urange)
 {
-	unsigned int		granularity =
-		bdev_discard_granularity(mp->m_ddev_targp->bt_bdev);
+	struct block_device	*bdev = mp->m_ddev_targp->bt_bdev;
 	struct block_device	*rt_bdev = NULL;
+	unsigned int		granularity = bdev_discard_granularity(bdev);
+	xfs_rfsblock_t		max_blocks = mp->m_sb.sb_dblocks;
 	struct fstrim_range	range;
 	xfs_daddr_t		start, end;
 	xfs_extlen_t		minlen;
-	xfs_rfsblock_t		max_blocks;
 	uint64_t		blocks_trimmed = 0;
 	int			error, last_error = 0;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
-	if (mp->m_rtdev_targp &&
-	    bdev_max_discard_sectors(mp->m_rtdev_targp->bt_bdev))
-		rt_bdev = mp->m_rtdev_targp->bt_bdev;
-	if (!bdev_max_discard_sectors(mp->m_ddev_targp->bt_bdev) && !rt_bdev)
-		return -EOPNOTSUPP;
-
-	if (rt_bdev)
-		granularity = max(granularity,
-				  bdev_discard_granularity(rt_bdev));
 
 	/*
 	 * We haven't recovered the log, so we cannot use our bnobt-guided
@@ -714,13 +705,36 @@ xfs_ioc_trim(
 	if (xfs_has_norecovery(mp))
 		return -EROFS;
 
+	/*
+	 * If the main device doesn't support discards, fail the entire ioctl
+	 * as the RT blocks are mapped after the main device blocks, and we'd
+	 * have to fake operation results for the main device for the ABI to
+	 * work.
+	 *
+	 * Note that while a main device that supports discards (SSD) and a RT
+	 * device that does not (HDD) is a fairly common setup, the reverse is
+	 * theoretically possible but rather odd, so this is not much of a loss.
+	 */
+	if (!bdev_max_discard_sectors(bdev))
+		return -EOPNOTSUPP;
+
+	if (mp->m_rtdev_targp) {
+		rt_bdev = mp->m_rtdev_targp->bt_bdev;
+		if (!bdev_max_discard_sectors(rt_bdev))
+			rt_bdev = NULL;
+	}
+	if (rt_bdev) {
+		max_blocks += mp->m_sb.sb_rblocks;
+		granularity =
+			max(granularity, bdev_discard_granularity(rt_bdev));
+	}
+
 	if (copy_from_user(&range, urange, sizeof(range)))
 		return -EFAULT;
 
 	range.minlen = max_t(u64, granularity, range.minlen);
 	minlen = XFS_B_TO_FSB(mp, range.minlen);
 
-	max_blocks = mp->m_sb.sb_dblocks + mp->m_sb.sb_rblocks;
 	if (range.start >= XFS_FSB_TO_B(mp, max_blocks) ||
 	    range.minlen > XFS_FSB_TO_B(mp, mp->m_ag_max_usable) ||
 	    range.len < mp->m_sb.sb_blocksize)
@@ -729,12 +743,10 @@ xfs_ioc_trim(
 	start = BTOBB(range.start);
 	end = start + BTOBBT(range.len) - 1;
 
-	if (bdev_max_discard_sectors(mp->m_ddev_targp->bt_bdev)) {
-		error = xfs_trim_datadev_extents(mp, start, end, minlen,
-				&blocks_trimmed);
-		if (error)
-			last_error = error;
-	}
+	error = xfs_trim_datadev_extents(mp, start, end, minlen,
+			&blocks_trimmed);
+	if (error)
+		last_error = error;
 
 	if (rt_bdev && !xfs_trim_should_stop()) {
 		error = xfs_trim_rtdev_extents(mp, start, end, minlen,
-- 
2.43.0

Re: [PATCH 2/2] xfs: don't extend the FITRIM range if the rt device does not support discard

[Darrick J. Wong]

On Fri, Aug 16, 2024 at 10:18:43AM +0200, Christoph Hellwig wrote:
> Fix the newly added discard support to only offer a FITRIM range that
> spans the RT device in addition to the main device if the RT device
> actually supports discard.  Without this we'll incorrectly accept
> a larger range than actually supported and confuse user space if the
> RT device does not support discard.  This can easily happen when the
> main device is a SSD but the RT device is a hard driver.
> 
> Move the code around a bit to keep the max_blocks and granularity
> assignments together and explicitly reject that case where only the
> RT device supports discard, as that does not fit the way the FITRIM ABI
> works very well and is a very fringe use case.

Is there more to this than generic/260 failing?  And if not, does the
following patch things up for you?

--D

From: Darrick J. Wong <djwong@kernel.org>
Subject: [PATCH] generic/260: fix for multi-device xfs with mixed discard support

Fix this test so that it can handle XFS filesystems with a realtime
volume when the data and rt devices do not both support discard.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
 common/rc  |   12 ++++++++++-
 common/xfs |   65 +++++++++++++++++++++++++++++++++++++++++++++++++-----------
 2 files changed, 64 insertions(+), 13 deletions(-)

diff --git a/common/rc b/common/rc
index b718030a59..426f2de43b 100644
--- a/common/rc
+++ b/common/rc
@@ -4207,6 +4207,16 @@ _require_batched_discard()
 	fi
 }
 
+_bdev_queue_property()
+{
+	local dev="$1"
+	local property="$2"
+	local default="$3"
+
+	local fname="/sys/block/$(_short_dev "$dev")/queue/$property"
+	cat "$fname" 2>/dev/null || echo "$default"
+}
+
 # Given a mountpoint and the device associated with that mountpoint, return the
 # maximum start offset that the FITRIM command will accept, in units of 1024
 # byte blocks.
@@ -4214,7 +4224,7 @@ _discard_max_offset_kb()
 {
 	case "$FSTYP" in
 	xfs)
-		_xfs_discard_max_offset_kb "$1"
+		_xfs_discard_max_offset_kb "$@"
 		;;
 	*)
 		$DF_PROG -k | awk -v dev="$2" -v mnt="$1" '$1 == dev && $7 == mnt { print $3 }'
diff --git a/common/xfs b/common/xfs
index 9501adac4c..5ad60a3ddc 100644
--- a/common/xfs
+++ b/common/xfs
@@ -1882,7 +1882,13 @@ _require_xfs_scratch_atomicswap()
 # of 1024 byte blocks.
 _xfs_discard_max_offset_kb()
 {
+	local mount="$1"
+	local dev="$2"
 	local statfs
+	local datadev_discard=
+	local rtdev_discard=
+	local dev_discard_max=
+	local rtdev_discard_max=
 
 	# Use awk to read the statfs output for the XFS filesystem, compute
 	# the two possible FITRIM offset maximums, and then use some horrid
@@ -1895,31 +1901,66 @@ _xfs_discard_max_offset_kb()
 	# 2: Realtime volume size in fsblocks.
 	# 3: Max FITRIM offset if we can only trim the data volume
 	# 4: Max FITRIM offset if we can trim the data and rt volumes
-	readarray -t statfs < <($XFS_IO_PROG -c 'statfs' "$1" | \
-		awk '{g[$1] = $3} END {printf("%d\n%d\n%d\n%d\n%d\n",
+	# 5: Max FITRIM offset if we can only trim the rt volume
+	readarray -t statfs < <($XFS_IO_PROG -c 'statfs' "$mount" | \
+		awk '{g[$1] = $3} END {printf("%d\n%d\n%d\n%d\n%d\n%d\n",
 			g["geom.bsize"],
 			g["geom.datablocks"],
 			g["geom.rtblocks"],
 			g["geom.bsize"] * g["geom.datablocks"] / 1024,
-			g["geom.bsize"] * (g["geom.datablocks"] + g["geom.rtblocks"]) / 1024);}')
+			g["geom.bsize"] * (g["geom.datablocks"] + g["geom.rtblocks"]) / 1024,
+			g["geom.bsize"] * g["geom.rtblocks"] / 1024);}')
 
 	# If the kernel supports discarding the realtime volume, then it will
 	# not reject a FITRIM for fsblock dblks+1, even if the len/minlen
 	# arguments are absurd.
 	if [ "${statfs[2]}" -gt 0 ]; then
-		if $FSTRIM_PROG -o "$((statfs[0] * statfs[1]))" \
+		case "$dev" in
+		"$SCRATCH_DEV")
+			rtdev_discard_max="$(_bdev_queue_property "$SCRATCH_RTDEV" discard_max_bytes 0)"
+			;;
+		"$TEST_DEV")
+			rtdev_discard_max="$(_bdev_queue_property "$TEST_RTDEV" discard_max_bytes 0)"
+			;;
+		*)
+			echo "Unrecognized device $dev" >&2
+			rtdev_discard_max=0
+			;;
+		esac
+
+		if [ "$rtdev_discard_max" -gt 0 ] &&
+		   $FSTRIM_PROG -o "$((statfs[0] * statfs[1]))" \
 				-l "${statfs[0]}" \
-				-m "$((statfs[0] * 2))" "$1" &>/dev/null; then
-			# The kernel supports discarding the rt volume, so
-			# print out the second answer from above.
-			echo "${statfs[4]}"
-			return
+				-m "$((statfs[0] * 2))" "$mount" &>/dev/null; then
+			# The kernel supports discarding the rt volume
+			rtdev_discard=2
 		fi
 	fi
 
-	# The kernel does not support discarding the rt volume or there is no
-	# rt volume.  Print out the first answer from above.
-	echo "${statfs[3]}"
+	# The kernel supports discarding the rt volume
+	dev_discard_max="$(_bdev_queue_property "$dev" discard_max_bytes 0)"
+	if [ "$dev_discard_max" -gt 0 ]; then
+		datadev_discard=1
+	fi
+
+	case "$datadev_discard$rtdev_discard" in
+	"12")
+		# Both devices support it
+		echo "${statfs[4]}"
+		;;
+	"1")
+		# Only the data device supports it
+		echo "${statfs[3]}"
+		;;
+	"2")
+		# Only the rt device supports it
+		echo "${statfs[5]}"
+		;;
+	*)
+		# No support at all
+		echo 0
+		;;
+	esac
 }
 
 # check if mkfs and the kernel support nocrc (v4) file systems

Re: [PATCH 2/2] xfs: don't extend the FITRIM range if the rt device does not support discard

[Christoph Hellwig]

On Fri, Aug 16, 2024 at 02:50:17PM -0700, Darrick J. Wong wrote:
> Is there more to this than generic/260 failing?

The only other users of the detection is generic/251, which doesn't
seem to care about the exact value.

> And if not, does the
> following patch things up for you?

This works.  OTOH it will break again with the zoned RT subvolume
which can't support FITRIM even on devices that claim it.  And for
actual users that care (and not just xfstests) these kinds of hacks
don't seem very palatable..

Re: [PATCH 2/2] xfs: don't extend the FITRIM range if the rt device does not support discard

[Darrick J. Wong]

On Mon, Aug 19, 2024 at 02:44:07PM +0200, Christoph Hellwig wrote:
> On Fri, Aug 16, 2024 at 02:50:17PM -0700, Darrick J. Wong wrote:
> > Is there more to this than generic/260 failing?
> 
> The only other users of the detection is generic/251, which doesn't
> seem to care about the exact value.
> 
> > And if not, does the
> > following patch things up for you?
> 
> This works.  OTOH it will break again with the zoned RT subvolume
> which can't support FITRIM even on devices that claim it.  And for
> actual users that care (and not just xfstests) these kinds of hacks
> don't seem very palatable..

What does discard do on a zoned device?  Is that how you reset the write
pointer?  And does that mean that either you tell the device to discard
everything it's written in a zone, or it will do nothing?

I see that this test really just puts a lower bound on how much FITRIM
can report that it discarded from an empty fs.  That number is already
rather meaningless on XFS because the amount "discarded" is roughly
(free space - (amount of pending gc work + discard work already in
progress)) and the user has no visibility into the amount of pending gc
work.  And you can repeatedly FITRIM and it reports the same number
since we have no idea if the device actually *did* anything.

Hmm.  No manpage for FITRIM.  Why don't we return the number of bytes
in the space map that we iterated as range.len?  Or perhaps leave it
unchanged?

--D

Re: [PATCH 2/2] xfs: don't extend the FITRIM range if the rt device does not support discard

[Christoph Hellwig]

On Mon, Aug 19, 2024 at 08:00:30AM -0700, Darrick J. Wong wrote:
> > This works.  OTOH it will break again with the zoned RT subvolume
> > which can't support FITRIM even on devices that claim it.  And for
> > actual users that care (and not just xfstests) these kinds of hacks
> > don't seem very palatable..
> 
> What does discard do on a zoned device?  Is that how you reset the write
> pointer?  And does that mean that either you tell the device to discard
> everything it's written in a zone, or it will do nothing?

On an actual zone device it will probably do nothing.  But at least for
NVMe the command used to implement discard is mandatory, so all
devices will show support.  We also support the zoned mode on
conventional devices, but instead of through FITRIM we want to issue
it instad of a zone reset when the whole rtg has been garbage collected.

> Hmm.  No manpage for FITRIM.  Why don't we return the number of bytes
> in the space map that we iterated as range.len?  Or perhaps leave it
> unchanged?

The above would seem sensible.  Not sure if we can still pull it
off, though.

Re: [PATCH 2/2] xfs: don't extend the FITRIM range if the rt device does not support discard

[Darrick J. Wong]

On Mon, Aug 19, 2024 at 05:08:04PM +0200, Christoph Hellwig wrote:
> On Mon, Aug 19, 2024 at 08:00:30AM -0700, Darrick J. Wong wrote:
> > > This works.  OTOH it will break again with the zoned RT subvolume
> > > which can't support FITRIM even on devices that claim it.  And for
> > > actual users that care (and not just xfstests) these kinds of hacks
> > > don't seem very palatable..
> > 
> > What does discard do on a zoned device?  Is that how you reset the write
> > pointer?  And does that mean that either you tell the device to discard
> > everything it's written in a zone, or it will do nothing?
> 
> On an actual zone device it will probably do nothing.  But at least for
> NVMe the command used to implement discard is mandatory, so all
> devices will show support.  We also support the zoned mode on
> conventional devices, but instead of through FITRIM we want to issue
> it instad of a zone reset when the whole rtg has been garbage collected.
> 
> > Hmm.  No manpage for FITRIM.  Why don't we return the number of bytes
> > in the space map that we iterated as range.len?  Or perhaps leave it
> > unchanged?
> 
> The above would seem sensible.  Not sure if we can still pull it
> off, though.

It seems to have survived testing on TOT overnight, so I'll bake it into
djwong-dev when I go through and remove the rtgroups/rtsb feature bits
today.  And I guess the rtgroups xarray conversion too.

--D

Re: [PATCH 2/2] xfs: don't extend the FITRIM range if the rt device does not support discard

[Christoph Hellwig]

On Tue, Aug 20, 2024 at 09:19:55AM -0700, Darrick J. Wong wrote:
> It seems to have survived testing on TOT overnight, so I'll bake it into
> djwong-dev

Note that this a fix for the new RT discard code in 6.11 and it would
be kinda nice to get the fix into the 6.11 tree so that we don't have
a release with the current state.