[PATCH v10 00/12] timekeeping/fs: multigrain timestamp redux

[PATCH v10 00/12] timekeeping/fs: multigrain timestamp redux

[Jeff Layton]

This is a replacement for the v6 series sitting in Christian's
vfs.mgtime branch. The main changes here are to the changelogs,
documentation and comments. I've also moved the timekeeping patches to
the front of the series, and done some minor cleanups.

The pipe1_threads test shows these averages on my test rig with this
series:

    v6.11:				89233600 (baseline)
    v6.11 + v9 series:			88460897 (<1% slower)

Thanks for all of the review so far!

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
Changes in v10:
- Reorder timekeeping patches to front of series
- Better separate timekeeping and fs changes
- add data_race annotations to per_cpu fetches
- get rid of pointless test for unsigned long to go negative
- better spacing for declarations and definitions in timekeeping_internal.h
- minor style cleanups
- Link to v9: https://lore.kernel.org/r/20241002-mgtime-v9-0-77e2baad57ac@kernel.org

Changes in v9:
- Comment and documentation cleanups.
- Drop the EXPORT_SYMBOL_GPL() from new timekeeper interfaces
- Link to v8: https://lore.kernel.org/r/20241001-mgtime-v8-0-903343d91bc3@kernel.org

Changes in v8:
- split patch that adds percpu counters into fs and timekeeping patches
- convert percpu counters to unsigned longs
- directly access the offs_real value in timekeeper instead of going
  through offsets array
- drop WARN_ON's in timekeeping patches
- better changelogs and more comments for the timekeeping bits
- better document how backward realtime clock jumps affect things
- Link to v7: https://lore.kernel.org/r/20240913-mgtime-v7-0-92d4020e3b00@kernel.org

Changes in v7:
- move the floor value handling into timekeeper for better performance
- Link to v6: https://lore.kernel.org/r/20240715-mgtime-v6-0-48e5d34bd2ba@kernel.org

Changes in v6:
- Normalize timespec64 in inode_set_ctime_to_ts
- use DEFINE_PER_CPU counters for better vfs consistency
- skip ctime cmpxchg if the result means nothing will change
- add trace_ctime_xchg_skip to track skipped ctime updates
- use __print_flags in ctime_ns_xchg tracepoint
- Link to v5: https://lore.kernel.org/r/20240711-mgtime-v5-0-37bb5b465feb@kernel.org

Changes in v5:
- refetch coarse time in coarse_ctime if not returning floor
- timestamp_truncate before swapping new ctime value into place
- track floor value as atomic64_t
- cleanups to Documentation file
- Link to v4: https://lore.kernel.org/r/20240708-mgtime-v4-0-a0f3c6fb57f3@kernel.org

Changes in v4:
- reordered tracepoint fields for better packing
- rework percpu counters again to also count fine grained timestamps
- switch to try_cmpxchg for better efficiency
- Link to v3: https://lore.kernel.org/r/20240705-mgtime-v3-0-85b2daa9b335@kernel.org

Changes in v3:
- Drop the conversion of i_ctime fields to ktime_t, and use an unused bit
  of the i_ctime_nsec field as QUERIED flag.
- Better tracepoints for tracking floor and ctime updates
- Reworked percpu counters to be more useful
- Track floor as monotonic value, which eliminates clock-jump problem

Changes in v2:
- Added Documentation file
- Link to v1: https://lore.kernel.org/r/20240626-mgtime-v1-0-a189352d0f8f@kernel.org

---
Jeff Layton (12):
      timekeeping: add interfaces for handling timestamps with a floor value
      timekeeping: add percpu counter for tracking floor swap events
      fs: add infrastructure for multigrain timestamps
      fs: have setattr_copy handle multigrain timestamps appropriately
      fs: handle delegated timestamps in setattr_copy_mgtime
      fs: tracepoints around multigrain timestamp events
      fs: add percpu counters for significant multigrain timestamp events
      Documentation: add a new file documenting multigrain timestamps
      xfs: switch to multigrain timestamps
      ext4: switch to multigrain timestamps
      btrfs: convert to multigrain timestamps
      tmpfs: add support for multigrain timestamps

 Documentation/filesystems/index.rst         |   1 +
 Documentation/filesystems/multigrain-ts.rst | 125 ++++++++++++
 fs/attr.c                                   |  60 +++++-
 fs/btrfs/file.c                             |  25 +--
 fs/btrfs/super.c                            |   3 +-
 fs/ext4/super.c                             |   2 +-
 fs/inode.c                                  | 282 +++++++++++++++++++++++++---
 fs/stat.c                                   |  46 ++++-
 fs/xfs/libxfs/xfs_trans_inode.c             |   6 +-
 fs/xfs/xfs_iops.c                           |  10 +-
 fs/xfs/xfs_super.c                          |   2 +-
 include/linux/fs.h                          |  36 +++-
 include/linux/timekeeping.h                 |   5 +
 include/trace/events/timestamp.h            | 124 ++++++++++++
 kernel/time/timekeeping.c                   | 106 +++++++++++
 kernel/time/timekeeping_debug.c             |  14 ++
 kernel/time/timekeeping_internal.h          |  15 ++
 mm/shmem.c                                  |   2 +-
 18 files changed, 791 insertions(+), 73 deletions(-)
---
base-commit: 98f7e32f20d28ec452afb208f9cffc08448a2652
change-id: 20240913-mgtime-20c98bcda88e

Best regards,
-- 
Jeff Layton <jlayton@kernel.org>

[PATCH v10 01/12] timekeeping: add interfaces for handling timestamps with a floor value

[Jeff Layton]

Multigrain timestamps allow the kernel to use fine-grained timestamps
when an inode's attributes is being actively observed via ->getattr().
With this support, it's possible for a file to get a fine-grained
timestamp, and another modified after it to get a coarse-grained stamp
that is earlier than the fine-grained time.  If this happens then the
files can appear to have been modified in reverse order, which breaks
VFS ordering guarantees [1].

To prevent this, maintain a floor value for multigrain timestamps.
Whenever a fine-grained timestamp is handed out, record it, and when
later coarse-grained stamps are handed out, ensure they are not earlier
than that value. If the coarse-grained timestamp is earlier than the
fine-grained floor, return the floor value instead.

Add a static singleton atomic64_t into timekeeper.c that is used to
keep track of the latest fine-grained time ever handed out. This is
tracked as a monotonic ktime_t value to ensure that it isn't affected by
clock jumps. Because it is updated at different times than the rest of
the timekeeper object, the floor value is managed independently of the
timekeeper via a cmpxchg() operation, and sits on its own cacheline.

Add two new public interfaces:

- ktime_get_coarse_real_ts64_mg() fills a timespec64 with the later of the
  coarse-grained clock and the floor time

- ktime_get_real_ts64_mg() gets the fine-grained clock value, and tries
  to swap it into the floor. A timespec64 is filled with the result.

The floor value is global and updated via a single try_cmpxchg(). If
that fails then the operation raced with a concurrent update. Any
concurrent update must be later than the existing floor value, so any
racing tasks can accept any resulting floor value without retrying.

[1]: POSIX requires that files be stamped with realtime clock values, and
     makes no provision for dealing with backward clock jumps. If a backward
     realtime clock jump occurs, then files can appear to have been modified
     in reverse order.

Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 include/linux/timekeeping.h |   4 ++
 kernel/time/timekeeping.c   | 105 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 109 insertions(+)

diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index fc12a9ba2c884271a75608211a72173b7ebaa24c..7aa85246c183576b039c02af4abba02b4a09ef9d 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -45,6 +45,10 @@ extern void ktime_get_real_ts64(struct timespec64 *tv);
 extern void ktime_get_coarse_ts64(struct timespec64 *ts);
 extern void ktime_get_coarse_real_ts64(struct timespec64 *ts);
 
+/* Multigrain timestamp interfaces */
+extern void ktime_get_coarse_real_ts64_mg(struct timespec64 *ts);
+extern void ktime_get_real_ts64_mg(struct timespec64 *ts);
+
 void getboottime64(struct timespec64 *ts);
 
 /*
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 5391e4167d60226dfc48c845170e36bcbeb7b292..ebfe846ebde35850c3e4d9c2cc45642c983d137f 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -114,6 +114,24 @@ static struct tk_fast tk_fast_raw  ____cacheline_aligned = {
 	.base[1] = FAST_TK_INIT,
 };
 
+/*
+ * Multigrain timestamps require tracking the latest fine-grained timestamp
+ * that has been issued, and never returning a coarse-grained timestamp that is
+ * earlier than that value.
+ *
+ * mg_floor represents the latest fine-grained time that has been handed out as
+ * a file timestamp on the system. This is tracked as a monotonic ktime_t, and
+ * converted to a realtime clock value on an as-needed basis.
+ *
+ * Maintaining mg_floor ensures the multigrain interfaces never issue a
+ * timestamp earlier than one that has been previously issued.
+ *
+ * The exception to this rule is when there is a backward realtime clock jump. If
+ * such an event occurs, a timestamp can appear to be earlier than a previous one.
+ */
+
+static __cacheline_aligned_in_smp atomic64_t mg_floor;
+
 static inline void tk_normalize_xtime(struct timekeeper *tk)
 {
 	while (tk->tkr_mono.xtime_nsec >= ((u64)NSEC_PER_SEC << tk->tkr_mono.shift)) {
@@ -2394,6 +2412,93 @@ void ktime_get_coarse_real_ts64(struct timespec64 *ts)
 }
 EXPORT_SYMBOL(ktime_get_coarse_real_ts64);
 
+/**
+ * ktime_get_coarse_real_ts64_mg - return latter of coarse grained time or floor
+ * @ts: timespec64 to be filled
+ *
+ * Fetch the global mg_floor value, convert it to realtime and
+ * compare it to the current coarse-grained time. Fill @ts with
+ * whichever is latest. Note that this is a filesystem-specific
+ * interface and should be avoided outside of that context.
+ */
+void ktime_get_coarse_real_ts64_mg(struct timespec64 *ts)
+{
+	struct timekeeper *tk = &tk_core.timekeeper;
+	u64 floor = atomic64_read(&mg_floor);
+	ktime_t f_real, offset, coarse;
+	unsigned int seq;
+
+	do {
+		seq = read_seqcount_begin(&tk_core.seq);
+		*ts = tk_xtime(tk);
+		offset = tk_core.timekeeper.offs_real;
+	} while (read_seqcount_retry(&tk_core.seq, seq));
+
+	coarse = timespec64_to_ktime(*ts);
+	f_real = ktime_add(floor, offset);
+	if (ktime_after(f_real, coarse))
+		*ts = ktime_to_timespec64(f_real);
+}
+
+/**
+ * ktime_get_real_ts64_mg - attempt to update floor value and return result
+ * @ts:		pointer to the timespec to be set
+ *
+ * Get a monotonic fine-grained time value and attempt to swap it into
+ * mg_floor. If that succeeds then accept the new floor value. If it fails
+ * then another task raced in during the interim time and updated the floor.
+ * Since any update to the floor must be later than the previous floor,
+ * either outcome is acceptable.
+ *
+ * Typically this will be called after calling ktime_get_coarse_real_ts64_mg(),
+ * and determining that the resulting coarse-grained timestamp did not effect
+ * a change in the ctime. Any more recent floor value would effect a change to
+ * the ctime, so there is no need to retry the atomic64_try_cmpxchg() on failure.
+ *
+ * @ts will be filled with the latest floor value, regardless of the outcome of
+ * the cmpxchg. Note that this is a filesystem specific interface and should be
+ * avoided outside of that context.
+ */
+void ktime_get_real_ts64_mg(struct timespec64 *ts)
+{
+	struct timekeeper *tk = &tk_core.timekeeper;
+	ktime_t old = atomic64_read(&mg_floor);
+	ktime_t offset, mono;
+	unsigned int seq;
+	u64 nsecs;
+
+	do {
+		seq = read_seqcount_begin(&tk_core.seq);
+
+		ts->tv_sec = tk->xtime_sec;
+		mono = tk->tkr_mono.base;
+		nsecs = timekeeping_get_ns(&tk->tkr_mono);
+		offset = tk_core.timekeeper.offs_real;
+	} while (read_seqcount_retry(&tk_core.seq, seq));
+
+	mono = ktime_add_ns(mono, nsecs);
+
+	/*
+	 * Attempt to update the floor with the new time value. As any
+	 * update must be later then the existing floor, and would effect
+	 * a change to the ctime from the perspective of the current task,
+	 * accept the resulting floor value regardless of the outcome of
+	 * the swap.
+	 */
+	if (atomic64_try_cmpxchg(&mg_floor, &old, mono)) {
+		ts->tv_nsec = 0;
+		timespec64_add_ns(ts, nsecs);
+	} else {
+		/*
+		 * Another task changed mg_floor since "old" was fetched.
+		 * "old" has been updated with the latest value of "mg_floor".
+		 * That value is newer than the previous floor value, which
+		 * is enough to effect a change to the ctime. Accept it.
+		 */
+		*ts = ktime_to_timespec64(ktime_add(old, offset));
+	}
+}
+
 void ktime_get_coarse_ts64(struct timespec64 *ts)
 {
 	struct timekeeper *tk = &tk_core.timekeeper;

-- 
2.46.2

[PATCH v10 02/12] timekeeping: add percpu counter for tracking floor swap events

[Jeff Layton]

The mgtime_floor value is a global variable for tracking the latest
fine-grained timestamp handed out. Because it's a global, track the
number of times that a new floor value is assigned.

Add a new percpu counter to the timekeeping code to track the number of
floor swap events that have occurred. A later patch will add a debugfs
file to display this counter alongside other stats involving multigrain
timestamps.

Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 include/linux/timekeeping.h        |  1 +
 kernel/time/timekeeping.c          |  1 +
 kernel/time/timekeeping_debug.c    | 14 ++++++++++++++
 kernel/time/timekeeping_internal.h | 15 +++++++++++++++
 4 files changed, 31 insertions(+)

diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index 7aa85246c183576b039c02af4abba02b4a09ef9d..84a035e86ac811f9e7b1649246b71c9296519149 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -48,6 +48,7 @@ extern void ktime_get_coarse_real_ts64(struct timespec64 *ts);
 /* Multigrain timestamp interfaces */
 extern void ktime_get_coarse_real_ts64_mg(struct timespec64 *ts);
 extern void ktime_get_real_ts64_mg(struct timespec64 *ts);
+extern unsigned long timekeeping_get_mg_floor_swaps(void);
 
 void getboottime64(struct timespec64 *ts);
 
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index ebfe846ebde35850c3e4d9c2cc45642c983d137f..e8b713e8ce5553f9e7de96c8e7c089714e0aa7a4 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -2488,6 +2488,7 @@ void ktime_get_real_ts64_mg(struct timespec64 *ts)
 	if (atomic64_try_cmpxchg(&mg_floor, &old, mono)) {
 		ts->tv_nsec = 0;
 		timespec64_add_ns(ts, nsecs);
+		timekeeping_inc_mg_floor_swaps();
 	} else {
 		/*
 		 * Another task changed mg_floor since "old" was fetched.
diff --git a/kernel/time/timekeeping_debug.c b/kernel/time/timekeeping_debug.c
index b73e8850e58d9c5b291559f475e67c7ed47c2db3..36d359cad7ca1d821bf42f59b3e50f89b14afd40 100644
--- a/kernel/time/timekeeping_debug.c
+++ b/kernel/time/timekeeping_debug.c
@@ -17,6 +17,9 @@
 
 #define NUM_BINS 32
 
+/* incremented every time mg_floor is updated */
+DEFINE_PER_CPU(unsigned long, timekeeping_mg_floor_swaps);
+
 static unsigned int sleep_time_bin[NUM_BINS] = {0};
 
 static int tk_debug_sleep_time_show(struct seq_file *s, void *data)
@@ -53,3 +56,14 @@ void tk_debug_account_sleep_time(const struct timespec64 *t)
 			   (s64)t->tv_sec, t->tv_nsec / NSEC_PER_MSEC);
 }
 
+unsigned long timekeeping_get_mg_floor_swaps(void)
+{
+	unsigned long sum = 0;
+	int cpu;
+
+	for_each_possible_cpu(cpu)
+		sum += data_race(per_cpu(timekeeping_mg_floor_swaps, cpu));
+
+	return sum;
+}
+
diff --git a/kernel/time/timekeeping_internal.h b/kernel/time/timekeeping_internal.h
index 4ca2787d1642e2f52bf985607ca3b03785cf9a50..0bbae825bc0226e4eed64e73fe3b454986c7573f 100644
--- a/kernel/time/timekeeping_internal.h
+++ b/kernel/time/timekeeping_internal.h
@@ -10,9 +10,24 @@
  * timekeeping debug functions
  */
 #ifdef CONFIG_DEBUG_FS
+
+DECLARE_PER_CPU(unsigned long, timekeeping_mg_floor_swaps);
+
+static inline void timekeeping_inc_mg_floor_swaps(void)
+{
+	this_cpu_inc(timekeeping_mg_floor_swaps);
+}
+
 extern void tk_debug_account_sleep_time(const struct timespec64 *t);
+
 #else
+
 #define tk_debug_account_sleep_time(x)
+
+static inline void timekeeping_inc_mg_floor_swaps(void)
+{
+}
+
 #endif
 
 #ifdef CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE

-- 
2.46.2

[PATCH v10 03/12] fs: add infrastructure for multigrain timestamps

[Jeff Layton]

The VFS has always used coarse-grained timestamps when updating the
ctime and mtime after a change. This has the benefit of allowing
filesystems to optimize away a lot metadata updates, down to around 1
per jiffy, even when a file is under heavy writes.

Unfortunately, this has always been an issue when we're exporting via
NFSv3, which relies on timestamps to validate caches. A lot of changes
can happen in a jiffy, so timestamps aren't sufficient to help the
client decide when to invalidate the cache. Even with NFSv4, a lot of
exported filesystems don't properly support a change attribute and are
subject to the same problems with timestamp granularity. Other
applications have similar issues with timestamps (e.g backup
applications).

If fine-grained timestamps were always used, that would improve the
situation, but that becomes rather expensive, as the underlying
filesystem would have to log a lot more metadata updates.

What is needed is a way to only use fine-grained timestamps when they
are being actively queried. Use the (unused) top bit in
inode->i_ctime_nsec as a flag that indicates whether the current
timestamps have been queried via stat() or the like. When it's set,
allow the update to use a fine-grained timestamp iff it's necessary to
make the ctime show a different value.

If it has been queried, then first see whether the current coarse time
is later than the existing ctime. If it is, accept that value.  If it
isn't, then get a fine-grained timestamp and attempt to stamp the inode
ctime with that value. If that races with another concurrent stamp, then
abandon the update and take the new value without retrying.

Filesystems can opt into this by setting the FS_MGTIME fstype flag.
Others should be unaffected (other than being subject to the same floor
value as multigrain filesystems).

Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/inode.c         | 139 +++++++++++++++++++++++++++++++++++++++++++----------
 fs/stat.c          |  43 ++++++++++++++++-
 include/linux/fs.h |  34 ++++++++++---
 3 files changed, 181 insertions(+), 35 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 10c4619faeef8cb81d84a91ec2d982d5a1a51a5c..53f56f6e1ff26e718080211880924f37cf0e5b3c 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2172,19 +2172,58 @@ int file_remove_privs(struct file *file)
 }
 EXPORT_SYMBOL(file_remove_privs);
 
+/**
+ * current_time - Return FS time (possibly fine-grained)
+ * @inode: inode.
+ *
+ * Return the current time truncated to the time granularity supported by
+ * the fs, as suitable for a ctime/mtime change. If the ctime is flagged
+ * as having been QUERIED, get a fine-grained timestamp, but don't update
+ * the floor.
+ *
+ * For a multigrain inode, this is effectively an estimate of the timestamp
+ * that a file would receive. An actual update must go through
+ * inode_set_ctime_current().
+ */
+struct timespec64 current_time(struct inode *inode)
+{
+	struct timespec64 now;
+	u32 cns;
+
+	ktime_get_coarse_real_ts64_mg(&now);
+
+	if (!is_mgtime(inode))
+		goto out;
+
+	/* If nothing has queried it, then coarse time is fine */
+	cns = smp_load_acquire(&inode->i_ctime_nsec);
+	if (cns & I_CTIME_QUERIED) {
+		/*
+		 * If there is no apparent change, then get a fine-grained
+		 * timestamp.
+		 */
+		if (now.tv_nsec == (cns & ~I_CTIME_QUERIED))
+			ktime_get_real_ts64(&now);
+	}
+out:
+	return timestamp_truncate(now, inode);
+}
+EXPORT_SYMBOL(current_time);
+
 static int inode_needs_update_time(struct inode *inode)
 {
+	struct timespec64 now, ts;
 	int sync_it = 0;
-	struct timespec64 now = current_time(inode);
-	struct timespec64 ts;
 
 	/* First try to exhaust all avenues to not sync */
 	if (IS_NOCMTIME(inode))
 		return 0;
 
+	now = current_time(inode);
+
 	ts = inode_get_mtime(inode);
 	if (!timespec64_equal(&ts, &now))
-		sync_it = S_MTIME;
+		sync_it |= S_MTIME;
 
 	ts = inode_get_ctime(inode);
 	if (!timespec64_equal(&ts, &now))
@@ -2562,6 +2601,15 @@ void inode_nohighmem(struct inode *inode)
 }
 EXPORT_SYMBOL(inode_nohighmem);
 
+struct timespec64 inode_set_ctime_to_ts(struct inode *inode, struct timespec64 ts)
+{
+	set_normalized_timespec64(&ts, ts.tv_sec, ts.tv_nsec);
+	inode->i_ctime_sec = ts.tv_sec;
+	inode->i_ctime_nsec = ts.tv_nsec;
+	return ts;
+}
+EXPORT_SYMBOL(inode_set_ctime_to_ts);
+
 /**
  * timestamp_truncate - Truncate timespec to a granularity
  * @t: Timespec
@@ -2594,36 +2642,77 @@ struct timespec64 timestamp_truncate(struct timespec64 t, struct inode *inode)
 EXPORT_SYMBOL(timestamp_truncate);
 
 /**
- * current_time - Return FS time
- * @inode: inode.
+ * inode_set_ctime_current - set the ctime to current_time
+ * @inode: inode
  *
- * Return the current time truncated to the time granularity supported by
- * the fs.
+ * Set the inode's ctime to the current value for the inode. Returns the
+ * current value that was assigned. If this is not a multigrain inode, then we
+ * set it to the later of the coarse time and floor value.
  *
- * Note that inode and inode->sb cannot be NULL.
- * Otherwise, the function warns and returns time without truncation.
+ * If it is multigrain, then we first see if the coarse-grained timestamp is
+ * distinct from what is already there. If so, then use that. Otherwise, get a
+ * fine-grained timestamp.
+ *
+ * After that, try to swap the new value into i_ctime_nsec. Accept the
+ * resulting ctime, regardless of the outcome of the swap. If it has
+ * already been replaced, then that timestamp is later than the earlier
+ * unacceptable one, and is thus acceptable.
  */
-struct timespec64 current_time(struct inode *inode)
+struct timespec64 inode_set_ctime_current(struct inode *inode)
 {
 	struct timespec64 now;
+	u32 cns, cur;
 
-	ktime_get_coarse_real_ts64(&now);
-	return timestamp_truncate(now, inode);
-}
-EXPORT_SYMBOL(current_time);
+	ktime_get_coarse_real_ts64_mg(&now);
+	now = timestamp_truncate(now, inode);
 
-/**
- * inode_set_ctime_current - set the ctime to current_time
- * @inode: inode
- *
- * Set the inode->i_ctime to the current value for the inode. Returns
- * the current value that was assigned to i_ctime.
- */
-struct timespec64 inode_set_ctime_current(struct inode *inode)
-{
-	struct timespec64 now = current_time(inode);
+	/* Just return that if this is not a multigrain fs */
+	if (!is_mgtime(inode)) {
+		inode_set_ctime_to_ts(inode, now);
+		goto out;
+	}
 
-	inode_set_ctime_to_ts(inode, now);
+	/*
+	 * A fine-grained time is only needed if someone has queried
+	 * for timestamps, and the current coarse grained time isn't
+	 * later than what's already there.
+	 */
+	cns = smp_load_acquire(&inode->i_ctime_nsec);
+	if (cns & I_CTIME_QUERIED) {
+		struct timespec64 ctime = { .tv_sec = inode->i_ctime_sec,
+					    .tv_nsec = cns & ~I_CTIME_QUERIED };
+
+		if (timespec64_compare(&now, &ctime) <= 0) {
+			ktime_get_real_ts64_mg(&now);
+			now = timestamp_truncate(now, inode);
+		}
+	}
+
+	/* No need to cmpxchg if it's exactly the same */
+	if (cns == now.tv_nsec && inode->i_ctime_sec == now.tv_sec)
+		goto out;
+	cur = cns;
+retry:
+	/* Try to swap the nsec value into place. */
+	if (try_cmpxchg(&inode->i_ctime_nsec, &cur, now.tv_nsec)) {
+		/* If swap occurred, then we're (mostly) done */
+		inode->i_ctime_sec = now.tv_sec;
+	} else {
+		/*
+		 * Was the change due to someone marking the old ctime QUERIED?
+		 * If so then retry the swap. This can only happen once since
+		 * the only way to clear I_CTIME_QUERIED is to stamp the inode
+		 * with a new ctime.
+		 */
+		if (!(cns & I_CTIME_QUERIED) && (cns | I_CTIME_QUERIED) == cur) {
+			cns = cur;
+			goto retry;
+		}
+		/* Otherwise, keep the existing ctime */
+		now.tv_sec = inode->i_ctime_sec;
+		now.tv_nsec = cur & ~I_CTIME_QUERIED;
+	}
+out:
 	return now;
 }
 EXPORT_SYMBOL(inode_set_ctime_current);
diff --git a/fs/stat.c b/fs/stat.c
index 89ce1be563108c1bc0ecabaff5b277258eb6c398..dd480bf51a2a764e5eb1d0a213c5ec8b640db911 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -26,6 +26,39 @@
 #include "internal.h"
 #include "mount.h"
 
+/**
+ * fill_mg_cmtime - Fill in the mtime and ctime and flag ctime as QUERIED
+ * @stat: where to store the resulting values
+ * @request_mask: STATX_* values requested
+ * @inode: inode from which to grab the c/mtime
+ *
+ * Given @inode, grab the ctime and mtime out if it and store the result
+ * in @stat. When fetching the value, flag it as QUERIED (if not already)
+ * so the next write will record a distinct timestamp.
+ *
+ * NB: The QUERIED flag is tracked in the ctime, but we set it there even
+ * if only the mtime was requested, as that ensures that the next mtime
+ * change will be distinct.
+ */
+void fill_mg_cmtime(struct kstat *stat, u32 request_mask, struct inode *inode)
+{
+	atomic_t *pcn = (atomic_t *)&inode->i_ctime_nsec;
+
+	/* If neither time was requested, then don't report them */
+	if (!(request_mask & (STATX_CTIME|STATX_MTIME))) {
+		stat->result_mask &= ~(STATX_CTIME|STATX_MTIME);
+		return;
+	}
+
+	stat->mtime = inode_get_mtime(inode);
+	stat->ctime.tv_sec = inode->i_ctime_sec;
+	stat->ctime.tv_nsec = (u32)atomic_read(pcn);
+	if (!(stat->ctime.tv_nsec & I_CTIME_QUERIED))
+		stat->ctime.tv_nsec = ((u32)atomic_fetch_or(I_CTIME_QUERIED, pcn));
+	stat->ctime.tv_nsec &= ~I_CTIME_QUERIED;
+}
+EXPORT_SYMBOL(fill_mg_cmtime);
+
 /**
  * generic_fillattr - Fill in the basic attributes from the inode struct
  * @idmap:		idmap of the mount the inode was found from
@@ -58,8 +91,14 @@ void generic_fillattr(struct mnt_idmap *idmap, u32 request_mask,
 	stat->rdev = inode->i_rdev;
 	stat->size = i_size_read(inode);
 	stat->atime = inode_get_atime(inode);
-	stat->mtime = inode_get_mtime(inode);
-	stat->ctime = inode_get_ctime(inode);
+
+	if (is_mgtime(inode)) {
+		fill_mg_cmtime(stat, request_mask, inode);
+	} else {
+		stat->ctime = inode_get_ctime(inode);
+		stat->mtime = inode_get_mtime(inode);
+	}
+
 	stat->blksize = i_blocksize(inode);
 	stat->blocks = inode->i_blocks;
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 6ca11e241a24950d4bd44954cb285d51da2751e9..eff688e75f2f29f1c44dca96370ee230f8c21db4 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1613,6 +1613,17 @@ static inline struct timespec64 inode_set_mtime(struct inode *inode,
 	return inode_set_mtime_to_ts(inode, ts);
 }
 
+/*
+ * Multigrain timestamps
+ *
+ * Conditionally use fine-grained ctime and mtime timestamps when there
+ * are users actively observing them via getattr. The primary use-case
+ * for this is NFS clients that use the ctime to distinguish between
+ * different states of the file, and that are often fooled by multiple
+ * operations that occur in the same coarse-grained timer tick.
+ */
+#define I_CTIME_QUERIED		((u32)BIT(31))
+
 static inline time64_t inode_get_ctime_sec(const struct inode *inode)
 {
 	return inode->i_ctime_sec;
@@ -1620,7 +1631,7 @@ static inline time64_t inode_get_ctime_sec(const struct inode *inode)
 
 static inline long inode_get_ctime_nsec(const struct inode *inode)
 {
-	return inode->i_ctime_nsec;
+	return inode->i_ctime_nsec & ~I_CTIME_QUERIED;
 }
 
 static inline struct timespec64 inode_get_ctime(const struct inode *inode)
@@ -1631,13 +1642,7 @@ static inline struct timespec64 inode_get_ctime(const struct inode *inode)
 	return ts;
 }
 
-static inline struct timespec64 inode_set_ctime_to_ts(struct inode *inode,
-						      struct timespec64 ts)
-{
-	inode->i_ctime_sec = ts.tv_sec;
-	inode->i_ctime_nsec = ts.tv_nsec;
-	return ts;
-}
+struct timespec64 inode_set_ctime_to_ts(struct inode *inode, struct timespec64 ts);
 
 /**
  * inode_set_ctime - set the ctime in the inode
@@ -2500,6 +2505,7 @@ struct file_system_type {
 #define FS_USERNS_MOUNT		8	/* Can be mounted by userns root */
 #define FS_DISALLOW_NOTIFY_PERM	16	/* Disable fanotify permission events */
 #define FS_ALLOW_IDMAP         32      /* FS has been updated to handle vfs idmappings. */
+#define FS_MGTIME		64	/* FS uses multigrain timestamps */
 #define FS_RENAME_DOES_D_MOVE	32768	/* FS will handle d_move() during rename() internally. */
 	int (*init_fs_context)(struct fs_context *);
 	const struct fs_parameter_spec *parameters;
@@ -2523,6 +2529,17 @@ struct file_system_type {
 
 #define MODULE_ALIAS_FS(NAME) MODULE_ALIAS("fs-" NAME)
 
+/**
+ * is_mgtime: is this inode using multigrain timestamps
+ * @inode: inode to test for multigrain timestamps
+ *
+ * Return true if the inode uses multigrain timestamps, false otherwise.
+ */
+static inline bool is_mgtime(const struct inode *inode)
+{
+	return inode->i_sb->s_type->fs_flags & FS_MGTIME;
+}
+
 extern struct dentry *mount_bdev(struct file_system_type *fs_type,
 	int flags, const char *dev_name, void *data,
 	int (*fill_super)(struct super_block *, void *, int));
@@ -3262,6 +3279,7 @@ extern void page_put_link(void *);
 extern int page_symlink(struct inode *inode, const char *symname, int len);
 extern const struct inode_operations page_symlink_inode_operations;
 extern void kfree_link(void *);
+void fill_mg_cmtime(struct kstat *stat, u32 request_mask, struct inode *inode);
 void generic_fillattr(struct mnt_idmap *, u32, struct inode *, struct kstat *);
 void generic_fill_statx_attr(struct inode *inode, struct kstat *stat);
 void generic_fill_statx_atomic_writes(struct kstat *stat,

-- 
2.46.2

[PATCH v10 04/12] fs: have setattr_copy handle multigrain timestamps appropriately

[Jeff Layton]

The setattr codepath is still using coarse-grained timestamps, even on
multigrain filesystems. To fix this, fetch the timestamp for ctime
updates later, at the point where the assignment occurs in setattr_copy.

On a multigrain inode, ignore the ia_ctime in the attrs, and always
update the ctime to the current clock value. Update the atime and mtime
with the same value (if needed) unless they are being set to other
specific values, a'la utimes().

Do not do this universally however, as some filesystems (e.g. most
networked fs) want to do an explicit update elsewhere before updating
the local inode.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/attr.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 46 insertions(+), 6 deletions(-)

diff --git a/fs/attr.c b/fs/attr.c
index c04d19b58f1224c2149da57e3224b7bbbc83561f..0309c2bd8afa04bc43db6ff207f8a58d9f6a617d 100644
--- a/fs/attr.c
+++ b/fs/attr.c
@@ -271,6 +271,42 @@ int inode_newsize_ok(const struct inode *inode, loff_t offset)
 }
 EXPORT_SYMBOL(inode_newsize_ok);
 
+/**
+ * setattr_copy_mgtime - update timestamps for mgtime inodes
+ * @inode: inode timestamps to be updated
+ * @attr: attrs for the update
+ *
+ * With multigrain timestamps, take more care to prevent races when
+ * updating the ctime. Always update the ctime to the very latest using
+ * the standard mechanism, and use that to populate the atime and mtime
+ * appropriately (unless those are being set to specific values).
+ */
+static void setattr_copy_mgtime(struct inode *inode, const struct iattr *attr)
+{
+	unsigned int ia_valid = attr->ia_valid;
+	struct timespec64 now;
+
+	/*
+	 * If the ctime isn't being updated then nothing else should be
+	 * either.
+	 */
+	if (!(ia_valid & ATTR_CTIME)) {
+		WARN_ON_ONCE(ia_valid & (ATTR_ATIME|ATTR_MTIME));
+		return;
+	}
+
+	now = inode_set_ctime_current(inode);
+	if (ia_valid & ATTR_ATIME_SET)
+		inode_set_atime_to_ts(inode, attr->ia_atime);
+	else if (ia_valid & ATTR_ATIME)
+		inode_set_atime_to_ts(inode, now);
+
+	if (ia_valid & ATTR_MTIME_SET)
+		inode_set_mtime_to_ts(inode, attr->ia_mtime);
+	else if (ia_valid & ATTR_MTIME)
+		inode_set_mtime_to_ts(inode, now);
+}
+
 /**
  * setattr_copy - copy simple metadata updates into the generic inode
  * @idmap:	idmap of the mount the inode was found from
@@ -303,12 +339,6 @@ void setattr_copy(struct mnt_idmap *idmap, struct inode *inode,
 
 	i_uid_update(idmap, attr, inode);
 	i_gid_update(idmap, attr, inode);
-	if (ia_valid & ATTR_ATIME)
-		inode_set_atime_to_ts(inode, attr->ia_atime);
-	if (ia_valid & ATTR_MTIME)
-		inode_set_mtime_to_ts(inode, attr->ia_mtime);
-	if (ia_valid & ATTR_CTIME)
-		inode_set_ctime_to_ts(inode, attr->ia_ctime);
 	if (ia_valid & ATTR_MODE) {
 		umode_t mode = attr->ia_mode;
 		if (!in_group_or_capable(idmap, inode,
@@ -316,6 +346,16 @@ void setattr_copy(struct mnt_idmap *idmap, struct inode *inode,
 			mode &= ~S_ISGID;
 		inode->i_mode = mode;
 	}
+
+	if (is_mgtime(inode))
+		return setattr_copy_mgtime(inode, attr);
+
+	if (ia_valid & ATTR_ATIME)
+		inode_set_atime_to_ts(inode, attr->ia_atime);
+	if (ia_valid & ATTR_MTIME)
+		inode_set_mtime_to_ts(inode, attr->ia_mtime);
+	if (ia_valid & ATTR_CTIME)
+		inode_set_ctime_to_ts(inode, attr->ia_ctime);
 }
 EXPORT_SYMBOL(setattr_copy);
 

-- 
2.46.2

[PATCH v10 05/12] fs: handle delegated timestamps in setattr_copy_mgtime

[Jeff Layton]

An update to the inode ctime typically requires the latest clock
value possible. The exception to this rule is when there is a nfsd write
delegation and the server is proxying timestamps from the client.

When nfsd gets a CB_GETATTR response, update the timestamp value in the
inode to the values that the client is tracking. The client doesn't send
a ctime value (since that's always determined by the exported
filesystem), but it can send a mtime value. In the case where it does,
update the ctime to a value commensurate with that instead of the
current time.

If ATTR_DELEG is set, then use ia_ctime value instead of setting the
timestamp to the current time.

With the addition of delegated timestamps, the server may receive a
request to update only the atime, which doesn't involve a ctime update.
Trust the ATTR_CTIME flag in the update and only update the ctime when
it's set.

Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/attr.c          | 28 +++++++++++++--------
 fs/inode.c         | 73 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/fs.h |  2 ++
 3 files changed, 93 insertions(+), 10 deletions(-)

diff --git a/fs/attr.c b/fs/attr.c
index 0309c2bd8afa04bc43db6ff207f8a58d9f6a617d..c614b954bda5244cc20ee82a98a8e68845f23bd7 100644
--- a/fs/attr.c
+++ b/fs/attr.c
@@ -286,16 +286,20 @@ static void setattr_copy_mgtime(struct inode *inode, const struct iattr *attr)
 	unsigned int ia_valid = attr->ia_valid;
 	struct timespec64 now;
 
-	/*
-	 * If the ctime isn't being updated then nothing else should be
-	 * either.
-	 */
-	if (!(ia_valid & ATTR_CTIME)) {
-		WARN_ON_ONCE(ia_valid & (ATTR_ATIME|ATTR_MTIME));
-		return;
+	if (ia_valid & ATTR_CTIME) {
+		/*
+		 * In the case of an update for a write delegation, we must respect
+		 * the value in ia_ctime and not use the current time.
+		 */
+		if (ia_valid & ATTR_DELEG)
+			now = inode_set_ctime_deleg(inode, attr->ia_ctime);
+		else
+			now = inode_set_ctime_current(inode);
+	} else {
+		/* If ATTR_CTIME isn't set, then ATTR_MTIME shouldn't be either. */
+		WARN_ON_ONCE(ia_valid & ATTR_MTIME);
 	}
 
-	now = inode_set_ctime_current(inode);
 	if (ia_valid & ATTR_ATIME_SET)
 		inode_set_atime_to_ts(inode, attr->ia_atime);
 	else if (ia_valid & ATTR_ATIME)
@@ -354,8 +358,12 @@ void setattr_copy(struct mnt_idmap *idmap, struct inode *inode,
 		inode_set_atime_to_ts(inode, attr->ia_atime);
 	if (ia_valid & ATTR_MTIME)
 		inode_set_mtime_to_ts(inode, attr->ia_mtime);
-	if (ia_valid & ATTR_CTIME)
-		inode_set_ctime_to_ts(inode, attr->ia_ctime);
+	if (ia_valid & ATTR_CTIME) {
+		if (ia_valid & ATTR_DELEG)
+			inode_set_ctime_deleg(inode, attr->ia_ctime);
+		else
+			inode_set_ctime_to_ts(inode, attr->ia_ctime);
+	}
 }
 EXPORT_SYMBOL(setattr_copy);
 
diff --git a/fs/inode.c b/fs/inode.c
index 53f56f6e1ff26e718080211880924f37cf0e5b3c..7d1ede60e549683502911f3bb3a3a079768e449b 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2717,6 +2717,79 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
 }
 EXPORT_SYMBOL(inode_set_ctime_current);
 
+/**
+ * inode_set_ctime_deleg - try to update the ctime on a delegated inode
+ * @inode: inode to update
+ * @update: timespec64 to set the ctime
+ *
+ * Attempt to atomically update the ctime on behalf of a delegation holder.
+ *
+ * The nfs server can call back the holder of a delegation to get updated
+ * inode attributes, including the mtime. When updating the mtime, update
+ * the ctime to a value at least equal to that.
+ *
+ * This can race with concurrent updates to the inode, in which
+ * case the update is skipped.
+ *
+ * Note that this works even when multigrain timestamps are not enabled,
+ * so it is used in either case.
+ */
+struct timespec64 inode_set_ctime_deleg(struct inode *inode, struct timespec64 update)
+{
+	struct timespec64 now, cur_ts;
+	u32 cur, old;
+
+	/* pairs with try_cmpxchg below */
+	cur = smp_load_acquire(&inode->i_ctime_nsec);
+	cur_ts.tv_nsec = cur & ~I_CTIME_QUERIED;
+	cur_ts.tv_sec = inode->i_ctime_sec;
+
+	/* If the update is older than the existing value, skip it. */
+	if (timespec64_compare(&update, &cur_ts) <= 0)
+		return cur_ts;
+
+	ktime_get_coarse_real_ts64_mg(&now);
+
+	/* Clamp the update to "now" if it's in the future */
+	if (timespec64_compare(&update, &now) > 0)
+		update = now;
+
+	update = timestamp_truncate(update, inode);
+
+	/* No need to update if the values are already the same */
+	if (timespec64_equal(&update, &cur_ts))
+		return cur_ts;
+
+	/*
+	 * Try to swap the nsec value into place. If it fails, that means
+	 * it raced with an update due to a write or similar activity. That
+	 * stamp takes precedence, so just skip the update.
+	 */
+retry:
+	old = cur;
+	if (try_cmpxchg(&inode->i_ctime_nsec, &cur, update.tv_nsec)) {
+		inode->i_ctime_sec = update.tv_sec;
+		mgtime_counter_inc(mg_ctime_swaps);
+		return update;
+	}
+
+	/*
+	 * Was the change due to another task marking the old ctime QUERIED?
+	 *
+	 * If so, then retry the swap. This can only happen once since
+	 * the only way to clear I_CTIME_QUERIED is to stamp the inode
+	 * with a new ctime.
+	 */
+	if (!(old & I_CTIME_QUERIED) && (cur == (old | I_CTIME_QUERIED)))
+		goto retry;
+
+	/* Otherwise, it was a new timestamp. */
+	cur_ts.tv_sec = inode->i_ctime_sec;
+	cur_ts.tv_nsec = cur & ~I_CTIME_QUERIED;
+	return cur_ts;
+}
+EXPORT_SYMBOL(inode_set_ctime_deleg);
+
 /**
  * in_group_or_capable - check whether caller is CAP_FSETID privileged
  * @idmap:	idmap of the mount @inode was found from
diff --git a/include/linux/fs.h b/include/linux/fs.h
index eff688e75f2f29f1c44dca96370ee230f8c21db4..ea7ed437d2b165debf680507aa450b2662fd5839 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1544,6 +1544,8 @@ static inline bool fsuidgid_has_mapping(struct super_block *sb,
 
 struct timespec64 current_time(struct inode *inode);
 struct timespec64 inode_set_ctime_current(struct inode *inode);
+struct timespec64 inode_set_ctime_deleg(struct inode *inode,
+					struct timespec64 update);
 
 static inline time64_t inode_get_atime_sec(const struct inode *inode)
 {

-- 
2.46.2

[PATCH v10 06/12] fs: tracepoints around multigrain timestamp events

[Jeff Layton]

Add some tracepoints around various multigrain timestamp events.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/inode.c                       |   9 ++-
 fs/stat.c                        |   3 +
 include/trace/events/timestamp.h | 124 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 135 insertions(+), 1 deletion(-)

diff --git a/fs/inode.c b/fs/inode.c
index 7d1ede60e549683502911f3bb3a3a079768e449b..f7a25c511d6b7069fa235135cf3bad0cda32815b 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -22,6 +22,9 @@
 #include <linux/iversion.h>
 #include <linux/rw_hint.h>
 #include <trace/events/writeback.h>
+#define CREATE_TRACE_POINTS
+#include <trace/events/timestamp.h>
+
 #include "internal.h"
 
 /*
@@ -2603,6 +2606,7 @@ EXPORT_SYMBOL(inode_nohighmem);
 
 struct timespec64 inode_set_ctime_to_ts(struct inode *inode, struct timespec64 ts)
 {
+	trace_inode_set_ctime_to_ts(inode, &ts);
 	set_normalized_timespec64(&ts, ts.tv_sec, ts.tv_nsec);
 	inode->i_ctime_sec = ts.tv_sec;
 	inode->i_ctime_nsec = ts.tv_nsec;
@@ -2689,14 +2693,17 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
 	}
 
 	/* No need to cmpxchg if it's exactly the same */
-	if (cns == now.tv_nsec && inode->i_ctime_sec == now.tv_sec)
+	if (cns == now.tv_nsec && inode->i_ctime_sec == now.tv_sec) {
+		trace_ctime_xchg_skip(inode, &now);
 		goto out;
+	}
 	cur = cns;
 retry:
 	/* Try to swap the nsec value into place. */
 	if (try_cmpxchg(&inode->i_ctime_nsec, &cur, now.tv_nsec)) {
 		/* If swap occurred, then we're (mostly) done */
 		inode->i_ctime_sec = now.tv_sec;
+		trace_ctime_ns_xchg(inode, cns, now.tv_nsec, cur);
 	} else {
 		/*
 		 * Was the change due to someone marking the old ctime QUERIED?
diff --git a/fs/stat.c b/fs/stat.c
index dd480bf51a2a764e5eb1d0a213c5ec8b640db911..6eb6c39d003755f9e602996ed93dcbd863847820 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -23,6 +23,8 @@
 #include <linux/uaccess.h>
 #include <asm/unistd.h>
 
+#include <trace/events/timestamp.h>
+
 #include "internal.h"
 #include "mount.h"
 
@@ -56,6 +58,7 @@ void fill_mg_cmtime(struct kstat *stat, u32 request_mask, struct inode *inode)
 	if (!(stat->ctime.tv_nsec & I_CTIME_QUERIED))
 		stat->ctime.tv_nsec = ((u32)atomic_fetch_or(I_CTIME_QUERIED, pcn));
 	stat->ctime.tv_nsec &= ~I_CTIME_QUERIED;
+	trace_fill_mg_cmtime(inode, &stat->ctime, &stat->mtime);
 }
 EXPORT_SYMBOL(fill_mg_cmtime);
 
diff --git a/include/trace/events/timestamp.h b/include/trace/events/timestamp.h
new file mode 100644
index 0000000000000000000000000000000000000000..c9e5ec930054887a6a7bae8e487611b5ded33d71
--- /dev/null
+++ b/include/trace/events/timestamp.h
@@ -0,0 +1,124 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM timestamp
+
+#if !defined(_TRACE_TIMESTAMP_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_TIMESTAMP_H
+
+#include <linux/tracepoint.h>
+#include <linux/fs.h>
+
+#define CTIME_QUERIED_FLAGS \
+	{ I_CTIME_QUERIED, "Q" }
+
+DECLARE_EVENT_CLASS(ctime,
+	TP_PROTO(struct inode *inode,
+		 struct timespec64 *ctime),
+
+	TP_ARGS(inode, ctime),
+
+	TP_STRUCT__entry(
+		__field(dev_t,		dev)
+		__field(ino_t,		ino)
+		__field(time64_t,	ctime_s)
+		__field(u32,		ctime_ns)
+		__field(u32,		gen)
+	),
+
+	TP_fast_assign(
+		__entry->dev		= inode->i_sb->s_dev;
+		__entry->ino		= inode->i_ino;
+		__entry->gen		= inode->i_generation;
+		__entry->ctime_s	= ctime->tv_sec;
+		__entry->ctime_ns	= ctime->tv_nsec;
+	),
+
+	TP_printk("ino=%d:%d:%ld:%u ctime=%lld.%u",
+		MAJOR(__entry->dev), MINOR(__entry->dev), __entry->ino, __entry->gen,
+		__entry->ctime_s, __entry->ctime_ns
+	)
+);
+
+DEFINE_EVENT(ctime, inode_set_ctime_to_ts,
+		TP_PROTO(struct inode *inode,
+			 struct timespec64 *ctime),
+		TP_ARGS(inode, ctime));
+
+DEFINE_EVENT(ctime, ctime_xchg_skip,
+		TP_PROTO(struct inode *inode,
+			 struct timespec64 *ctime),
+		TP_ARGS(inode, ctime));
+
+TRACE_EVENT(ctime_ns_xchg,
+	TP_PROTO(struct inode *inode,
+		 u32 old,
+		 u32 new,
+		 u32 cur),
+
+	TP_ARGS(inode, old, new, cur),
+
+	TP_STRUCT__entry(
+		__field(dev_t,		dev)
+		__field(ino_t,		ino)
+		__field(u32,		gen)
+		__field(u32,		old)
+		__field(u32,		new)
+		__field(u32,		cur)
+	),
+
+	TP_fast_assign(
+		__entry->dev		= inode->i_sb->s_dev;
+		__entry->ino		= inode->i_ino;
+		__entry->gen		= inode->i_generation;
+		__entry->old		= old;
+		__entry->new		= new;
+		__entry->cur		= cur;
+	),
+
+	TP_printk("ino=%d:%d:%ld:%u old=%u:%s new=%u cur=%u:%s",
+		MAJOR(__entry->dev), MINOR(__entry->dev), __entry->ino, __entry->gen,
+		__entry->old & ~I_CTIME_QUERIED,
+		__print_flags(__entry->old & I_CTIME_QUERIED, "|", CTIME_QUERIED_FLAGS),
+		__entry->new,
+		__entry->cur & ~I_CTIME_QUERIED,
+		__print_flags(__entry->cur & I_CTIME_QUERIED, "|", CTIME_QUERIED_FLAGS)
+	)
+);
+
+TRACE_EVENT(fill_mg_cmtime,
+	TP_PROTO(struct inode *inode,
+		 struct timespec64 *ctime,
+		 struct timespec64 *mtime),
+
+	TP_ARGS(inode, ctime, mtime),
+
+	TP_STRUCT__entry(
+		__field(dev_t,		dev)
+		__field(ino_t,		ino)
+		__field(time64_t,	ctime_s)
+		__field(time64_t,	mtime_s)
+		__field(u32,		ctime_ns)
+		__field(u32,		mtime_ns)
+		__field(u32,		gen)
+	),
+
+	TP_fast_assign(
+		__entry->dev		= inode->i_sb->s_dev;
+		__entry->ino		= inode->i_ino;
+		__entry->gen		= inode->i_generation;
+		__entry->ctime_s	= ctime->tv_sec;
+		__entry->mtime_s	= mtime->tv_sec;
+		__entry->ctime_ns	= ctime->tv_nsec;
+		__entry->mtime_ns	= mtime->tv_nsec;
+	),
+
+	TP_printk("ino=%d:%d:%ld:%u ctime=%lld.%u mtime=%lld.%u",
+		MAJOR(__entry->dev), MINOR(__entry->dev), __entry->ino, __entry->gen,
+		__entry->ctime_s, __entry->ctime_ns,
+		__entry->mtime_s, __entry->mtime_ns
+	)
+);
+#endif /* _TRACE_TIMESTAMP_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>

-- 
2.46.2

[PATCH v10 07/12] fs: add percpu counters for significant multigrain timestamp events

[Jeff Layton]

New percpu counters for counting various stats around multigrain
timestamp events, and a new debugfs file for displaying them when
CONFIG_DEBUG_FS is enabled:

- number of attempted ctime updates
- number of successful i_ctime_nsec swaps
- number of fine-grained timestamp fetches
- number of floor value swap events

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/inode.c | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)

diff --git a/fs/inode.c b/fs/inode.c
index f7a25c511d6b7069fa235135cf3bad0cda32815b..6d501c7308aefcbb8001d64cb46e57f1839b8a3b 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -21,6 +21,8 @@
 #include <linux/list_lru.h>
 #include <linux/iversion.h>
 #include <linux/rw_hint.h>
+#include <linux/seq_file.h>
+#include <linux/debugfs.h>
 #include <trace/events/writeback.h>
 #define CREATE_TRACE_POINTS
 #include <trace/events/timestamp.h>
@@ -101,6 +103,70 @@ long get_nr_dirty_inodes(void)
 	return nr_dirty > 0 ? nr_dirty : 0;
 }
 
+#ifdef CONFIG_DEBUG_FS
+static DEFINE_PER_CPU(long, mg_ctime_updates);
+static DEFINE_PER_CPU(long, mg_fine_stamps);
+static DEFINE_PER_CPU(long, mg_ctime_swaps);
+
+static unsigned long get_mg_ctime_updates(void)
+{
+	unsigned long sum = 0;
+	int i;
+
+	for_each_possible_cpu(i)
+		sum += data_race(per_cpu(mg_ctime_updates, i));
+	return sum;
+}
+
+static unsigned long get_mg_fine_stamps(void)
+{
+	unsigned long sum = 0;
+	int i;
+
+	for_each_possible_cpu(i)
+		sum += data_race(per_cpu(mg_fine_stamps, i));
+	return sum;
+}
+
+static unsigned long get_mg_ctime_swaps(void)
+{
+	unsigned long sum = 0;
+	int i;
+
+	for_each_possible_cpu(i)
+		sum += data_race(per_cpu(mg_ctime_swaps, i));
+	return sum;
+}
+
+#define mgtime_counter_inc(__var)	this_cpu_inc(__var)
+
+static int mgts_show(struct seq_file *s, void *p)
+{
+	unsigned long ctime_updates = get_mg_ctime_updates();
+	unsigned long ctime_swaps = get_mg_ctime_swaps();
+	unsigned long fine_stamps = get_mg_fine_stamps();
+	unsigned long floor_swaps = timekeeping_get_mg_floor_swaps();
+
+	seq_printf(s, "%lu %lu %lu %lu\n",
+		   ctime_updates, ctime_swaps, fine_stamps, floor_swaps);
+	return 0;
+}
+
+DEFINE_SHOW_ATTRIBUTE(mgts);
+
+static int __init mg_debugfs_init(void)
+{
+	debugfs_create_file("multigrain_timestamps", S_IFREG | S_IRUGO, NULL, NULL, &mgts_fops);
+	return 0;
+}
+late_initcall(mg_debugfs_init);
+
+#else /* ! CONFIG_DEBUG_FS */
+
+#define mgtime_counter_inc(__var)	do { } while (0)
+
+#endif /* CONFIG_DEBUG_FS */
+
 /*
  * Handle nr_inode sysctl
  */
@@ -2689,8 +2755,10 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
 		if (timespec64_compare(&now, &ctime) <= 0) {
 			ktime_get_real_ts64_mg(&now);
 			now = timestamp_truncate(now, inode);
+			mgtime_counter_inc(mg_fine_stamps);
 		}
 	}
+	mgtime_counter_inc(mg_ctime_updates);
 
 	/* No need to cmpxchg if it's exactly the same */
 	if (cns == now.tv_nsec && inode->i_ctime_sec == now.tv_sec) {
@@ -2704,6 +2772,7 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
 		/* If swap occurred, then we're (mostly) done */
 		inode->i_ctime_sec = now.tv_sec;
 		trace_ctime_ns_xchg(inode, cns, now.tv_nsec, cur);
+		mgtime_counter_inc(mg_ctime_swaps);
 	} else {
 		/*
 		 * Was the change due to someone marking the old ctime QUERIED?

-- 
2.46.2

[PATCH v10 08/12] Documentation: add a new file documenting multigrain timestamps

[Jeff Layton]

Add a high-level document that describes how multigrain timestamps work,
rationale for them, and some info about implementation and tradeoffs.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 Documentation/filesystems/index.rst         |   1 +
 Documentation/filesystems/multigrain-ts.rst | 125 ++++++++++++++++++++++++++++
 2 files changed, 126 insertions(+)

diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index e8e496d23e1dd5b523889159b464d7adf5d5c30a..44e9e77ffe0d4b9c85f9921190d33dfd21acff8f 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -29,6 +29,7 @@ algorithms work.
    fiemap
    files
    locks
+   multigrain-ts
    mount_api
    quota
    seq_file
diff --git a/Documentation/filesystems/multigrain-ts.rst b/Documentation/filesystems/multigrain-ts.rst
new file mode 100644
index 0000000000000000000000000000000000000000..c779e47284e80f54ad9fc8a6a0b03228dbbf3d59
--- /dev/null
+++ b/Documentation/filesystems/multigrain-ts.rst
@@ -0,0 +1,125 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================
+Multigrain Timestamps
+=====================
+
+Introduction
+============
+Historically, the kernel has always used coarse time values to stamp inodes.
+This value is updated every jiffy, so any change that happens within that jiffy
+will end up with the same timestamp.
+
+When the kernel goes to stamp an inode (due to a read or write), it first gets
+the current time and then compares it to the existing timestamp(s) to see
+whether anything will change. If nothing changed, then it can avoid updating
+the inode's metadata.
+
+Coarse timestamps are therefore good from a performance standpoint, since they
+reduce the need for metadata updates, but bad from the standpoint of
+determining whether anything has changed, since a lot of things can happen in a
+jiffy.
+
+They are particularly troublesome with NFSv3, where unchanging timestamps can
+make it difficult to tell whether to invalidate caches. NFSv4 provides a
+dedicated change attribute that should always show a visible change, but not
+all filesystems implement this properly, causing the NFS server to substitute
+the ctime in many cases.
+
+Multigrain timestamps aim to remedy this by selectively using fine-grained
+timestamps when a file has had its timestamps queried recently, and the current
+coarse-grained time does not cause a change.
+
+Inode Timestamps
+================
+There are currently 3 timestamps in the inode that are updated to the current
+wallclock time on different activity:
+
+ctime:
+  The inode change time. This is stamped with the current time whenever
+  the inode's metadata is changed. Note that this value is not settable
+  from userland.
+
+mtime:
+  The inode modification time. This is stamped with the current time
+  any time a file's contents change.
+
+atime:
+  The inode access time. This is stamped whenever an inode's contents are
+  read. Widely considered to be a terrible mistake. Usually avoided with
+  options like noatime or relatime.
+
+Updating the mtime always implies a change to the ctime, but updating the
+atime due to a read request does not.
+
+Multigrain timestamps are only tracked for the ctime and the mtime. atimes are
+not affected and always use the coarse-grained value (subject to the floor).
+
+Inode Timestamp Ordering
+========================
+
+In addition to just providing info about changes to individual files, file
+timestamps also serve an important purpose in applications like "make". These
+programs measure timestamps in order to determine whether source files might be
+newer than cached objects.
+
+Userland applications like make can only determine ordering based on
+operational boundaries. For a syscall those are the syscall entry and exit
+points. For io_uring or nfsd operations, that's the request submission and
+response. In the case of concurrent operations, userland can make no
+determination about the order in which things will occur.
+
+For instance, if a single thread modifies one file, and then another file in
+sequence, the second file must show an equal or later mtime than the first. The
+same is true if two threads are issuing similar operations that do not overlap
+in time.
+
+If however, two threads have racing syscalls that overlap in time, then there
+is no such guarantee, and the second file may appear to have been modified
+before, after or at the same time as the first, regardless of which one was
+submitted first.
+
+Note that the above assumes that the system doesn't experience a backward jump
+of the realtime clock. If that occurs at an inopportune time, then timestamps
+can appear to go backward, even on a properly functioning system.
+
+Multigrain Timestamp Implementation
+===================================
+Multigrain timestamps are aimed at ensuring that changes to a single file are
+always recognizable, without violating the ordering guarantees when multiple
+different files are modified. This affects the mtime and the ctime, but the
+atime will always use coarse-grained timestamps.
+
+It uses an unused bit in the i_ctime_nsec field to indicate whether the mtime
+or ctime has been queried. If either or both have, then the kernel takes
+special care to ensure the next timestamp update will display a visible change.
+This ensures tight cache coherency for use-cases like NFS, without sacrificing
+the benefits of reduced metadata updates when files aren't being watched.
+
+The Ctime Floor Value
+=====================
+It's not sufficient to simply use fine or coarse-grained timestamps based on
+whether the mtime or ctime has been queried. A file could get a fine grained
+timestamp, and then a second file modified later could get a coarse-grained one
+that appears earlier than the first, which would break the kernel's timestamp
+ordering guarantees.
+
+To mitigate this problem, maintain a global floor value that ensures that
+this can't happen. The two files in the above example may appear to have been
+modified at the same time in such a case, but they will never show the reverse
+order. To avoid problems with realtime clock jumps, the floor is managed as a
+monotonic ktime_t, and the values are converted to realtime clock values as
+needed.
+
+Implementation Notes
+====================
+Multigrain timestamps are intended for use by local filesystems that get
+ctime values from the local clock. This is in contrast to network filesystems
+and the like that just mirror timestamp values from a server.
+
+For most filesystems, it's sufficient to just set the FS_MGTIME flag in the
+fstype->fs_flags in order to opt-in, providing the ctime is only ever set via
+inode_set_ctime_current(). If the filesystem has a ->getattr routine that
+doesn't call generic_fillattr, then it should call fill_mg_cmtime() to
+fill those values. For setattr, it should use setattr_copy() to update the
+timestamps, or otherwise mimic its behavior.

-- 
2.46.2

[PATCH v10 09/12] xfs: switch to multigrain timestamps

[Jeff Layton]

Enable multigrain timestamps, which should ensure that there is an
apparent change to the timestamp whenever it has been written after
being actively observed via getattr.

Also, anytime the mtime changes, the ctime must also change, and those
are now the only two options for xfs_trans_ichgtime. Have that function
unconditionally bump the ctime, and ASSERT that XFS_ICHGTIME_CHG is
always set.

Finally, stop setting STATX_CHANGE_COOKIE in getattr, since the ctime
should give us better semantics now.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/xfs/libxfs/xfs_trans_inode.c |  6 +++---
 fs/xfs/xfs_iops.c               | 10 +++-------
 fs/xfs/xfs_super.c              |  2 +-
 3 files changed, 7 insertions(+), 11 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_trans_inode.c b/fs/xfs/libxfs/xfs_trans_inode.c
index 3c40f37e82c73cf871bb252553331b60c6b1973b..c962ad64b0c10058c1e2eecf664fbc67ec7302f2 100644
--- a/fs/xfs/libxfs/xfs_trans_inode.c
+++ b/fs/xfs/libxfs/xfs_trans_inode.c
@@ -62,12 +62,12 @@ xfs_trans_ichgtime(
 	ASSERT(tp);
 	xfs_assert_ilocked(ip, XFS_ILOCK_EXCL);
 
-	tv = current_time(inode);
+	/* If the mtime changes, then ctime must also change */
+	ASSERT(flags & XFS_ICHGTIME_CHG);
 
+	tv = inode_set_ctime_current(inode);
 	if (flags & XFS_ICHGTIME_MOD)
 		inode_set_mtime_to_ts(inode, tv);
-	if (flags & XFS_ICHGTIME_CHG)
-		inode_set_ctime_to_ts(inode, tv);
 	if (flags & XFS_ICHGTIME_ACCESS)
 		inode_set_atime_to_ts(inode, tv);
 	if (flags & XFS_ICHGTIME_CREATE)
diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index 1cdc8034f54d93f4a40ab3e3e4f91c6c9dfed7ec..a1c4a350a6dbfd19028ccecdfdb271879f769ccb 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -597,8 +597,9 @@ xfs_vn_getattr(
 	stat->gid = vfsgid_into_kgid(vfsgid);
 	stat->ino = ip->i_ino;
 	stat->atime = inode_get_atime(inode);
-	stat->mtime = inode_get_mtime(inode);
-	stat->ctime = inode_get_ctime(inode);
+
+	fill_mg_cmtime(stat, request_mask, inode);
+
 	stat->blocks = XFS_FSB_TO_BB(mp, ip->i_nblocks + ip->i_delayed_blks);
 
 	if (xfs_has_v3inodes(mp)) {
@@ -608,11 +609,6 @@ xfs_vn_getattr(
 		}
 	}
 
-	if ((request_mask & STATX_CHANGE_COOKIE) && IS_I_VERSION(inode)) {
-		stat->change_cookie = inode_query_iversion(inode);
-		stat->result_mask |= STATX_CHANGE_COOKIE;
-	}
-
 	/*
 	 * Note: If you add another clause to set an attribute flag, please
 	 * update attributes_mask below.
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 27e9f749c4c7fc75c3385aba2e02ac9fc5d1719d..210481b03fdb48fd50e9a7a109d8bcee0e7e3a29 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -2052,7 +2052,7 @@ static struct file_system_type xfs_fs_type = {
 	.init_fs_context	= xfs_init_fs_context,
 	.parameters		= xfs_fs_parameters,
 	.kill_sb		= xfs_kill_sb,
-	.fs_flags		= FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
+	.fs_flags		= FS_REQUIRES_DEV | FS_ALLOW_IDMAP | FS_MGTIME,
 };
 MODULE_ALIAS_FS("xfs");
 

-- 
2.46.2

[PATCH v10 10/12] ext4: switch to multigrain timestamps

[Jeff Layton]

Enable multigrain timestamps, which should ensure that there is an
apparent change to the timestamp whenever it has been written after
being actively observed via getattr.

For ext4, we only need to enable the FS_MGTIME flag.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/ext4/super.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index e72145c4ae5a071cf4a809d0519a01a8fb84dc2d..a125d9435b8a1c8f7a96a2a0bdd9ce1b4671f8a2 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -7298,7 +7298,7 @@ static struct file_system_type ext4_fs_type = {
 	.init_fs_context	= ext4_init_fs_context,
 	.parameters		= ext4_param_specs,
 	.kill_sb		= ext4_kill_sb,
-	.fs_flags		= FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
+	.fs_flags		= FS_REQUIRES_DEV | FS_ALLOW_IDMAP | FS_MGTIME,
 };
 MODULE_ALIAS_FS("ext4");
 

-- 
2.46.2

[PATCH v10 11/12] btrfs: convert to multigrain timestamps

[Jeff Layton]

Enable multigrain timestamps, which should ensure that there is an
apparent change to the timestamp whenever it has been written after
being actively observed via getattr.

Beyond enabling the FS_MGTIME flag, this patch eliminates
update_time_for_write, which goes to great pains to avoid in-memory
stores. Just have it overwrite the timestamps unconditionally.

Note that this also drops the IS_I_VERSION check and unconditionally
bumps the change attribute, since SB_I_VERSION is always set on btrfs.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/btrfs/file.c  | 25 ++++---------------------
 fs/btrfs/super.c |  3 ++-
 2 files changed, 6 insertions(+), 22 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 2aeb8116549ca970432042a315f29d9e7fa00980..1656ad7498b8161ec94a2752b0ab7cb723fada1c 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1120,26 +1120,6 @@ void btrfs_check_nocow_unlock(struct btrfs_inode *inode)
 	btrfs_drew_write_unlock(&inode->root->snapshot_lock);
 }
 
-static void update_time_for_write(struct inode *inode)
-{
-	struct timespec64 now, ts;
-
-	if (IS_NOCMTIME(inode))
-		return;
-
-	now = current_time(inode);
-	ts = inode_get_mtime(inode);
-	if (!timespec64_equal(&ts, &now))
-		inode_set_mtime_to_ts(inode, now);
-
-	ts = inode_get_ctime(inode);
-	if (!timespec64_equal(&ts, &now))
-		inode_set_ctime_to_ts(inode, now);
-
-	if (IS_I_VERSION(inode))
-		inode_inc_iversion(inode);
-}
-
 int btrfs_write_check(struct kiocb *iocb, struct iov_iter *from, size_t count)
 {
 	struct file *file = iocb->ki_filp;
@@ -1170,7 +1150,10 @@ int btrfs_write_check(struct kiocb *iocb, struct iov_iter *from, size_t count)
 	 * need to start yet another transaction to update the inode as we will
 	 * update the inode when we finish writing whatever data we write.
 	 */
-	update_time_for_write(inode);
+	if (!IS_NOCMTIME(inode)) {
+		inode_set_mtime_to_ts(inode, inode_set_ctime_current(inode));
+		inode_inc_iversion(inode);
+	}
 
 	start_pos = round_down(pos, fs_info->sectorsize);
 	oldsize = i_size_read(inode);
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 98fa0f382480a2a51420d586b1e2a2fa6c58d025..d423acfe11d0d1702ff1e17a87d11d65d3ce8cdb 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2198,7 +2198,8 @@ static struct file_system_type btrfs_fs_type = {
 	.init_fs_context	= btrfs_init_fs_context,
 	.parameters		= btrfs_fs_parameters,
 	.kill_sb		= btrfs_kill_super,
-	.fs_flags		= FS_REQUIRES_DEV | FS_BINARY_MOUNTDATA | FS_ALLOW_IDMAP,
+	.fs_flags		= FS_REQUIRES_DEV | FS_BINARY_MOUNTDATA |
+				  FS_ALLOW_IDMAP | FS_MGTIME,
  };
 
 MODULE_ALIAS_FS("btrfs");

-- 
2.46.2

[PATCH v10 12/12] tmpfs: add support for multigrain timestamps

[Jeff Layton]

Enable multigrain timestamps, which should ensure that there is an
apparent change to the timestamp whenever it has been written after
being actively observed via getattr.

tmpfs only requires the FS_MGTIME flag.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 mm/shmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 5a77acf6ac6a621dc7b5e7b46402b2b714b45bea..5f17eaaa32e2902228be7b245c5b3b11c5fb6a56 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -4804,7 +4804,7 @@ static struct file_system_type shmem_fs_type = {
 	.parameters	= shmem_fs_parameters,
 #endif
 	.kill_sb	= kill_litter_super,
-	.fs_flags	= FS_USERNS_MOUNT | FS_ALLOW_IDMAP,
+	.fs_flags	= FS_USERNS_MOUNT | FS_ALLOW_IDMAP | FS_MGTIME,
 };
 
 void __init shmem_init(void)

-- 
2.46.2

Re: (subset) [PATCH v10 00/12] timekeeping/fs: multigrain timestamp redux

[Christian Brauner]

On Wed, 02 Oct 2024 17:27:15 -0400, Jeff Layton wrote:
> This is a replacement for the v6 series sitting in Christian's
> vfs.mgtime branch. The main changes here are to the changelogs,
> documentation and comments. I've also moved the timekeeping patches to
> the front of the series, and done some minor cleanups.
> 
> The pipe1_threads test shows these averages on my test rig with this
> series:
> 
> [...]

I've merged the tag that Thomas provided with the time specific changes and
pulled the remaining patches - excluding 01/12 and 02/12.

---

Applied to the vfs.mgtime branch of the vfs/vfs.git tree.
Patches in the vfs.mgtime branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs.mgtime

[03/12] fs: add infrastructure for multigrain timestamps
        https://git.kernel.org/vfs/vfs/c/4e40eff0b573
[04/12] fs: have setattr_copy handle multigrain timestamps appropriately
        https://git.kernel.org/vfs/vfs/c/b82f92d5dd1a
[05/12] fs: handle delegated timestamps in setattr_copy_mgtime
        https://git.kernel.org/vfs/vfs/c/d8d11298e8a1
[06/12] fs: tracepoints around multigrain timestamp events
        https://git.kernel.org/vfs/vfs/c/a80f53809ccc
[07/12] fs: add percpu counters for significant multigrain timestamp events
        https://git.kernel.org/vfs/vfs/c/7b1aba010c47
[08/12] Documentation: add a new file documenting multigrain timestamps
        https://git.kernel.org/vfs/vfs/c/95c6907be544
[09/12] xfs: switch to multigrain timestamps
        https://git.kernel.org/vfs/vfs/c/0f4865448420
[10/12] ext4: switch to multigrain timestamps
        https://git.kernel.org/vfs/vfs/c/e44ab3151adc
[11/12] btrfs: convert to multigrain timestamps
        https://git.kernel.org/vfs/vfs/c/0d4f9f7ad685
[12/12] tmpfs: add support for multigrain timestamps
        https://git.kernel.org/vfs/vfs/c/cba2a92eff80