[PATCH 6.1 CANDIDATE v2 0/4] xfs inodegc fixes for 6.1.y (from v6.4)

[PATCH 6.1 CANDIDATE v2 0/4] xfs inodegc fixes for 6.1.y (from v6.4)

[Amir Goldstein]

Darrick,

These are the patches we discussed that Leah requested for the 5.15.y
backport of non-blocking inodegc pushes series [1].

They may or may not help the 5.15.y -> 6.1.y regression that was
reported by Chris [2].

This v2 series has gone through 3 rounds of kdevops loop on top
of the testing already run on v1.

Please ACK.

Thanks,
Amir.

Changed since v1:
- include: 2d5f38a31980 ("xfs: disable reaping in fscounters scrub")

[1] https://www.spinics.net/lists/linux-xfs/msg61813.html
[2] https://lore.kernel.org/all/ZK4E%2FgGuaBu+qvKL@dread.disaster.area/

Darrick J. Wong (4):
  xfs: explicitly specify cpu when forcing inodegc delayed work to run
    immediately
  xfs: check that per-cpu inodegc workers actually run on that cpu
  xfs: disable reaping in fscounters scrub
  xfs: fix xfs_inodegc_stop racing with mod_delayed_work

 fs/xfs/scrub/common.c     | 26 -------------------------
 fs/xfs/scrub/common.h     |  2 --
 fs/xfs/scrub/fscounters.c | 13 ++++++-------
 fs/xfs/scrub/scrub.c      |  2 --
 fs/xfs/scrub/scrub.h      |  1 -
 fs/xfs/xfs_icache.c       | 40 ++++++++++++++++++++++++++++++++-------
 fs/xfs/xfs_mount.h        |  3 +++
 fs/xfs/xfs_super.c        |  3 +++
 8 files changed, 45 insertions(+), 45 deletions(-)

-- 
2.34.1

[PATCH 6.1 CANDIDATE v2 1/4] xfs: explicitly specify cpu when forcing inodegc delayed work to run immediately

[Amir Goldstein]

From: "Darrick J. Wong" <djwong@kernel.org>

commit 03e0add80f4cf3f7393edb574eeb3a89a1db7758 upstream.

I've been noticing odd racing behavior in the inodegc code that could
only be explained by one cpu adding an inode to its inactivation llist
at the same time that another cpu is processing that cpu's llist.
Preemption is disabled between get/put_cpu_ptr, so the only explanation
is scheduler mayhem.  I inserted the following debug code into
xfs_inodegc_worker (see the next patch):

	ASSERT(gc->cpu == smp_processor_id());

This assertion tripped during overnight tests on the arm64 machines, but
curiously not on x86_64.  I think we haven't observed any resource leaks
here because the lockfree list code can handle simultaneous llist_add
and llist_del_all functions operating on the same list.  However, the
whole point of having percpu inodegc lists is to take advantage of warm
memory caches by inactivating inodes on the last processor to touch the
inode.

The incorrect scheduling seems to occur after an inodegc worker is
subjected to mod_delayed_work().  This wraps mod_delayed_work_on with
WORK_CPU_UNBOUND specified as the cpu number.  Unbound allows for
scheduling on any cpu, not necessarily the same one that scheduled the
work.

Because preemption is disabled for as long as we have the gc pointer, I
think it's safe to use current_cpu() (aka smp_processor_id) to queue the
delayed work item on the correct cpu.

Fixes: 7cf2b0f9611b ("xfs: bound maximum wait time for inodegc work")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/xfs/xfs_icache.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
index eae7427062cf..536885f8b8a8 100644
--- a/fs/xfs/xfs_icache.c
+++ b/fs/xfs/xfs_icache.c
@@ -2052,7 +2052,8 @@ xfs_inodegc_queue(
 		queue_delay = 0;
 
 	trace_xfs_inodegc_queue(mp, __return_address);
-	mod_delayed_work(mp->m_inodegc_wq, &gc->work, queue_delay);
+	mod_delayed_work_on(current_cpu(), mp->m_inodegc_wq, &gc->work,
+			queue_delay);
 	put_cpu_ptr(gc);
 
 	if (xfs_inodegc_want_flush_work(ip, items, shrinker_hits)) {
@@ -2096,7 +2097,8 @@ xfs_inodegc_cpu_dead(
 
 	if (xfs_is_inodegc_enabled(mp)) {
 		trace_xfs_inodegc_queue(mp, __return_address);
-		mod_delayed_work(mp->m_inodegc_wq, &gc->work, 0);
+		mod_delayed_work_on(current_cpu(), mp->m_inodegc_wq, &gc->work,
+				0);
 	}
 	put_cpu_ptr(gc);
 }
-- 
2.34.1

[PATCH 6.1 CANDIDATE v2 2/4] xfs: check that per-cpu inodegc workers actually run on that cpu

[Amir Goldstein]

From: "Darrick J. Wong" <djwong@kernel.org>

commit b37c4c8339cd394ea6b8b415026603320a185651 upstream.

Now that we've allegedly worked out the problem of the per-cpu inodegc
workers being scheduled on the wrong cpu, let's put in a debugging knob
to let us know if a worker ever gets mis-scheduled again.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/xfs/xfs_icache.c | 2 ++
 fs/xfs/xfs_mount.h  | 3 +++
 fs/xfs/xfs_super.c  | 3 +++
 3 files changed, 8 insertions(+)

diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
index 536885f8b8a8..7ce262dcabca 100644
--- a/fs/xfs/xfs_icache.c
+++ b/fs/xfs/xfs_icache.c
@@ -1848,6 +1848,8 @@ xfs_inodegc_worker(
 	struct llist_node	*node = llist_del_all(&gc->list);
 	struct xfs_inode	*ip, *n;
 
+	ASSERT(gc->cpu == smp_processor_id());
+
 	WRITE_ONCE(gc->items, 0);
 
 	if (!node)
diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index 8aca2cc173ac..69ddd5319634 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -66,6 +66,9 @@ struct xfs_inodegc {
 	/* approximate count of inodes in the list */
 	unsigned int		items;
 	unsigned int		shrinker_hits;
+#if defined(DEBUG) || defined(XFS_WARN)
+	unsigned int		cpu;
+#endif
 };
 
 /*
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index ee4b429a2f2c..4b179526913f 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1084,6 +1084,9 @@ xfs_inodegc_init_percpu(
 
 	for_each_possible_cpu(cpu) {
 		gc = per_cpu_ptr(mp->m_inodegc, cpu);
+#if defined(DEBUG) || defined(XFS_WARN)
+		gc->cpu = cpu;
+#endif
 		init_llist_head(&gc->list);
 		gc->items = 0;
 		INIT_DELAYED_WORK(&gc->work, xfs_inodegc_worker);
-- 
2.34.1

[PATCH 6.1 CANDIDATE v2 3/4] xfs: disable reaping in fscounters scrub

[Amir Goldstein]

From: "Darrick J. Wong" <djwong@kernel.org>

commit 2d5f38a31980d7090f5bf91021488dc61a0ba8ee upstream.

The fscounters scrub code doesn't work properly because it cannot
quiesce updates to the percpu counters in the filesystem, hence it
returns false corruption reports.  This has been fixed properly in
one of the online repair patchsets that are under review by replacing
the xchk_disable_reaping calls with an exclusive filesystem freeze.
Disabling background gc isn't sufficient to fix the problem.

In other words, scrub doesn't need to call xfs_inodegc_stop, which is
just as well since it wasn't correct to allow scrub to call
xfs_inodegc_start when something else could be calling xfs_inodegc_stop
(e.g. trying to freeze the filesystem).

Neuter the scrubber for now, and remove the xchk_*_reaping functions.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/xfs/scrub/common.c     | 26 --------------------------
 fs/xfs/scrub/common.h     |  2 --
 fs/xfs/scrub/fscounters.c | 13 ++++++-------
 fs/xfs/scrub/scrub.c      |  2 --
 fs/xfs/scrub/scrub.h      |  1 -
 5 files changed, 6 insertions(+), 38 deletions(-)

diff --git a/fs/xfs/scrub/common.c b/fs/xfs/scrub/common.c
index 9bbbf20f401b..e71449658ecc 100644
--- a/fs/xfs/scrub/common.c
+++ b/fs/xfs/scrub/common.c
@@ -865,29 +865,3 @@ xchk_ilock_inverted(
 	}
 	return -EDEADLOCK;
 }
-
-/* Pause background reaping of resources. */
-void
-xchk_stop_reaping(
-	struct xfs_scrub	*sc)
-{
-	sc->flags |= XCHK_REAPING_DISABLED;
-	xfs_blockgc_stop(sc->mp);
-	xfs_inodegc_stop(sc->mp);
-}
-
-/* Restart background reaping of resources. */
-void
-xchk_start_reaping(
-	struct xfs_scrub	*sc)
-{
-	/*
-	 * Readonly filesystems do not perform inactivation or speculative
-	 * preallocation, so there's no need to restart the workers.
-	 */
-	if (!xfs_is_readonly(sc->mp)) {
-		xfs_inodegc_start(sc->mp);
-		xfs_blockgc_start(sc->mp);
-	}
-	sc->flags &= ~XCHK_REAPING_DISABLED;
-}
diff --git a/fs/xfs/scrub/common.h b/fs/xfs/scrub/common.h
index 454145db10e7..2ca80102e704 100644
--- a/fs/xfs/scrub/common.h
+++ b/fs/xfs/scrub/common.h
@@ -148,7 +148,5 @@ static inline bool xchk_skip_xref(struct xfs_scrub_metadata *sm)
 
 int xchk_metadata_inode_forks(struct xfs_scrub *sc);
 int xchk_ilock_inverted(struct xfs_inode *ip, uint lock_mode);
-void xchk_stop_reaping(struct xfs_scrub *sc);
-void xchk_start_reaping(struct xfs_scrub *sc);
 
 #endif	/* __XFS_SCRUB_COMMON_H__ */
diff --git a/fs/xfs/scrub/fscounters.c b/fs/xfs/scrub/fscounters.c
index 6a6f8fe7f87c..88d6961e3886 100644
--- a/fs/xfs/scrub/fscounters.c
+++ b/fs/xfs/scrub/fscounters.c
@@ -128,13 +128,6 @@ xchk_setup_fscounters(
 	if (error)
 		return error;
 
-	/*
-	 * Pause background reclaim while we're scrubbing to reduce the
-	 * likelihood of background perturbations to the counters throwing off
-	 * our calculations.
-	 */
-	xchk_stop_reaping(sc);
-
 	return xchk_trans_alloc(sc, 0);
 }
 
@@ -353,6 +346,12 @@ xchk_fscounters(
 	if (fdblocks > mp->m_sb.sb_dblocks)
 		xchk_set_corrupt(sc);
 
+	/*
+	 * XXX: We can't quiesce percpu counter updates, so exit early.
+	 * This can be re-enabled when we gain exclusive freeze functionality.
+	 */
+	return 0;
+
 	/*
 	 * If ifree exceeds icount by more than the minimum variance then
 	 * something's probably wrong with the counters.
diff --git a/fs/xfs/scrub/scrub.c b/fs/xfs/scrub/scrub.c
index 2e8e400f10a9..95132490fda5 100644
--- a/fs/xfs/scrub/scrub.c
+++ b/fs/xfs/scrub/scrub.c
@@ -171,8 +171,6 @@ xchk_teardown(
 	}
 	if (sc->sm->sm_flags & XFS_SCRUB_IFLAG_REPAIR)
 		mnt_drop_write_file(sc->file);
-	if (sc->flags & XCHK_REAPING_DISABLED)
-		xchk_start_reaping(sc);
 	if (sc->buf) {
 		kmem_free(sc->buf);
 		sc->buf = NULL;
diff --git a/fs/xfs/scrub/scrub.h b/fs/xfs/scrub/scrub.h
index 3de5287e98d8..4cb32c27df10 100644
--- a/fs/xfs/scrub/scrub.h
+++ b/fs/xfs/scrub/scrub.h
@@ -88,7 +88,6 @@ struct xfs_scrub {
 
 /* XCHK state flags grow up from zero, XREP state flags grown down from 2^31 */
 #define XCHK_TRY_HARDER		(1 << 0)  /* can't get resources, try again */
-#define XCHK_REAPING_DISABLED	(1 << 2)  /* background block reaping paused */
 #define XREP_ALREADY_FIXED	(1 << 31) /* checking our repair work */
 
 /* Metadata scrubbers */
-- 
2.34.1

[PATCH 6.1 CANDIDATE v2 4/4] xfs: fix xfs_inodegc_stop racing with mod_delayed_work

[Amir Goldstein]

From: "Darrick J. Wong" <djwong@kernel.org>

commit 2254a7396a0ca6309854948ee1c0a33fa4268cec upstream.

syzbot reported this warning from the faux inodegc shrinker that tries
to kick off inodegc work:

------------[ cut here ]------------
WARNING: CPU: 1 PID: 102 at kernel/workqueue.c:1445 __queue_work+0xd44/0x1120 kernel/workqueue.c:1444
RIP: 0010:__queue_work+0xd44/0x1120 kernel/workqueue.c:1444
Call Trace:
 __queue_delayed_work+0x1c8/0x270 kernel/workqueue.c:1672
 mod_delayed_work_on+0xe1/0x220 kernel/workqueue.c:1746
 xfs_inodegc_shrinker_scan fs/xfs/xfs_icache.c:2212 [inline]
 xfs_inodegc_shrinker_scan+0x250/0x4f0 fs/xfs/xfs_icache.c:2191
 do_shrink_slab+0x428/0xaa0 mm/vmscan.c:853
 shrink_slab+0x175/0x660 mm/vmscan.c:1013
 shrink_one+0x502/0x810 mm/vmscan.c:5343
 shrink_many mm/vmscan.c:5394 [inline]
 lru_gen_shrink_node mm/vmscan.c:5511 [inline]
 shrink_node+0x2064/0x35f0 mm/vmscan.c:6459
 kswapd_shrink_node mm/vmscan.c:7262 [inline]
 balance_pgdat+0xa02/0x1ac0 mm/vmscan.c:7452
 kswapd+0x677/0xd60 mm/vmscan.c:7712
 kthread+0x2e8/0x3a0 kernel/kthread.c:376
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:308

This warning corresponds to this code in __queue_work:

	/*
	 * For a draining wq, only works from the same workqueue are
	 * allowed. The __WQ_DESTROYING helps to spot the issue that
	 * queues a new work item to a wq after destroy_workqueue(wq).
	 */
	if (unlikely(wq->flags & (__WQ_DESTROYING | __WQ_DRAINING) &&
		     WARN_ON_ONCE(!is_chained_work(wq))))
		return;

For this to trip, we must have a thread draining the inodedgc workqueue
and a second thread trying to queue inodegc work to that workqueue.
This can happen if freezing or a ro remount race with reclaim poking our
faux inodegc shrinker and another thread dropping an unlinked O_RDONLY
file:

Thread 0	Thread 1	Thread 2

xfs_inodegc_stop

				xfs_inodegc_shrinker_scan
				xfs_is_inodegc_enabled
				<yes, will continue>

xfs_clear_inodegc_enabled
xfs_inodegc_queue_all
<list empty, do not queue inodegc worker>

		xfs_inodegc_queue
		<add to list>
		xfs_is_inodegc_enabled
		<no, returns>

drain_workqueue
<set WQ_DRAINING>

				llist_empty
				<no, will queue list>
				mod_delayed_work_on(..., 0)
				__queue_work
				<sees WQ_DRAINING, kaboom>

In other words, everything between the access to inodegc_enabled state
and the decision to poke the inodegc workqueue requires some kind of
coordination to avoid the WQ_DRAINING state.  We could perhaps introduce
a lock here, but we could also try to eliminate WQ_DRAINING from the
picture.

We could replace the drain_workqueue call with a loop that flushes the
workqueue and queues workers as long as there is at least one inode
present in the per-cpu inodegc llists.  We've disabled inodegc at this
point, so we know that the number of queued inodes will eventually hit
zero as long as xfs_inodegc_start cannot reactivate the workers.

There are four callers of xfs_inodegc_start.  Three of them come from the
VFS with s_umount held: filesystem thawing, failed filesystem freezing,
and the rw remount transition.  The fourth caller is mounting rw (no
remount or freezing possible).

There are three callers ofs xfs_inodegc_stop.  One is unmounting (no
remount or thaw possible).  Two of them come from the VFS with s_umount
held: fs freezing and ro remount transition.

Hence, it is correct to replace the drain_workqueue call with a loop
that drains the inodegc llists.

Fixes: 6191cf3ad59f ("xfs: flush inodegc workqueue tasks before cancel")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/xfs/xfs_icache.c | 32 +++++++++++++++++++++++++++-----
 1 file changed, 27 insertions(+), 5 deletions(-)

diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
index 7ce262dcabca..d884cba1d707 100644
--- a/fs/xfs/xfs_icache.c
+++ b/fs/xfs/xfs_icache.c
@@ -431,18 +431,23 @@ xfs_iget_check_free_state(
 }
 
 /* Make all pending inactivation work start immediately. */
-static void
+static bool
 xfs_inodegc_queue_all(
 	struct xfs_mount	*mp)
 {
 	struct xfs_inodegc	*gc;
 	int			cpu;
+	bool			ret = false;
 
 	for_each_online_cpu(cpu) {
 		gc = per_cpu_ptr(mp->m_inodegc, cpu);
-		if (!llist_empty(&gc->list))
+		if (!llist_empty(&gc->list)) {
 			mod_delayed_work_on(cpu, mp->m_inodegc_wq, &gc->work, 0);
+			ret = true;
+		}
 	}
+
+	return ret;
 }
 
 /*
@@ -1894,24 +1899,41 @@ xfs_inodegc_flush(
 
 /*
  * Flush all the pending work and then disable the inode inactivation background
- * workers and wait for them to stop.
+ * workers and wait for them to stop.  Caller must hold sb->s_umount to
+ * coordinate changes in the inodegc_enabled state.
  */
 void
 xfs_inodegc_stop(
 	struct xfs_mount	*mp)
 {
+	bool			rerun;
+
 	if (!xfs_clear_inodegc_enabled(mp))
 		return;
 
+	/*
+	 * Drain all pending inodegc work, including inodes that could be
+	 * queued by racing xfs_inodegc_queue or xfs_inodegc_shrinker_scan
+	 * threads that sample the inodegc state just prior to us clearing it.
+	 * The inodegc flag state prevents new threads from queuing more
+	 * inodes, so we queue pending work items and flush the workqueue until
+	 * all inodegc lists are empty.  IOWs, we cannot use drain_workqueue
+	 * here because it does not allow other unserialized mechanisms to
+	 * reschedule inodegc work while this draining is in progress.
+	 */
 	xfs_inodegc_queue_all(mp);
-	drain_workqueue(mp->m_inodegc_wq);
+	do {
+		flush_workqueue(mp->m_inodegc_wq);
+		rerun = xfs_inodegc_queue_all(mp);
+	} while (rerun);
 
 	trace_xfs_inodegc_stop(mp, __return_address);
 }
 
 /*
  * Enable the inode inactivation background workers and schedule deferred inode
- * inactivation work if there is any.
+ * inactivation work if there is any.  Caller must hold sb->s_umount to
+ * coordinate changes in the inodegc_enabled state.
  */
 void
 xfs_inodegc_start(
-- 
2.34.1

Re: [PATCH 6.1 CANDIDATE v2 0/4] xfs inodegc fixes for 6.1.y (from v6.4)

[Darrick J. Wong]

On Fri, Jul 14, 2023 at 09:45:05AM +0300, Amir Goldstein wrote:
> Darrick,
> 
> These are the patches we discussed that Leah requested for the 5.15.y
> backport of non-blocking inodegc pushes series [1].
> 
> They may or may not help the 5.15.y -> 6.1.y regression that was
> reported by Chris [2].
> 
> This v2 series has gone through 3 rounds of kdevops loop on top
> of the testing already run on v1.
> 
> Please ACK.

Looks good to me,
Acked-by: Darrick J. Wong <djwong@kernel.org>

--D

> 
> Thanks,
> Amir.
> 
> Changed since v1:
> - include: 2d5f38a31980 ("xfs: disable reaping in fscounters scrub")
> 
> [1] https://www.spinics.net/lists/linux-xfs/msg61813.html
> [2] https://lore.kernel.org/all/ZK4E%2FgGuaBu+qvKL@dread.disaster.area/
> 
> Darrick J. Wong (4):
>   xfs: explicitly specify cpu when forcing inodegc delayed work to run
>     immediately
>   xfs: check that per-cpu inodegc workers actually run on that cpu
>   xfs: disable reaping in fscounters scrub
>   xfs: fix xfs_inodegc_stop racing with mod_delayed_work
> 
>  fs/xfs/scrub/common.c     | 26 -------------------------
>  fs/xfs/scrub/common.h     |  2 --
>  fs/xfs/scrub/fscounters.c | 13 ++++++-------
>  fs/xfs/scrub/scrub.c      |  2 --
>  fs/xfs/scrub/scrub.h      |  1 -
>  fs/xfs/xfs_icache.c       | 40 ++++++++++++++++++++++++++++++++-------
>  fs/xfs/xfs_mount.h        |  3 +++
>  fs/xfs/xfs_super.c        |  3 +++
>  8 files changed, 45 insertions(+), 45 deletions(-)
> 
> -- 
> 2.34.1
>