[PATCH v2 00/11] fs: multigrain timestamp redux

[PATCH v2 00/11] fs: multigrain timestamp redux

[Jeff Layton]

This set is essentially unchanged from the last one, aside from the
new file in Documentation/. I had a review comment from Andi Kleen
suggesting that the ctime_floor should be per time_namespace, but I
think that's incorrect as the realtime clock is not namespaced.

At LSF/MM this year, we had a discussion about the inode change
attribute. At the time I mentioned that I thought I could salvage the
multigrain timestamp work that had to be reverted last year [1].  That
version had to be reverted because it was possible for a file to get a
coarse grained timestamp that appeared to be earlier than another file
that had recently gotten a fine-grained stamp.

This version corrects the problem by establishing a per-time_namespace
ctime_floor value that should prevent this from occurring. In the above
situation that was problematic before, the two files might end up with
the same timestamp value, but they won't appear to have been modified in
the wrong order.

That problem was discovered by the test-stat-time gnulib test. Note that
that test still fails on multigrain timestamps, but that's because its
method of determining the minimum delay that will show a timestamp
change will no longer work with multigrain timestamps. I have a patch to
change the testcase to use a different method that I've posted to the
bug-gnulib mailing list.

The big question with this set is whether the performance will be
suitable. The testing I've done seems to show performance parity with
multigrain timestamps enabled, but it's hard to rule this out regressing
some workload.

This set is based on top of Christian's vfs.misc branch (which has the
earlier change to track inode timestamps as discrete integers). If there
are no major objections, I'd like to let this soak in linux-next for a
bit to see if any problems shake out.

[1]: https://lore.kernel.org/linux-fsdevel/20230807-mgctime-v7-0-d1dec143a704@kernel.org/

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
Changes in v2:
- Added Documentation file
- Link to v1: https://lore.kernel.org/r/20240626-mgtime-v1-0-a189352d0f8f@kernel.org

---
Jeff Layton (11):
      fs: turn inode ctime fields into a single ktime_t
      fs: uninline inode_get_ctime and inode_set_ctime_to_ts
      fs: tracepoints for inode_needs_update_time and inode_set_ctime_to_ts
      fs: add infrastructure for multigrain timestamps
      fs: add percpu counters to count fine vs. coarse timestamps
      fs: have setattr_copy handle multigrain timestamps appropriately
      xfs: switch to multigrain timestamps
      ext4: switch to multigrain timestamps
      btrfs: convert to multigrain timestamps
      tmpfs: add support for multigrain timestamps
      Documentation: add a new file documenting multigrain timestamps

 Documentation/filesystems/multigrain-ts.rst | 126 ++++++++++++++++
 fs/attr.c                                   |  52 ++++++-
 fs/btrfs/file.c                             |  25 +---
 fs/btrfs/super.c                            |   3 +-
 fs/ext4/super.c                             |   2 +-
 fs/inode.c                                  | 221 +++++++++++++++++++++++++---
 fs/stat.c                                   |  39 ++++-
 fs/xfs/libxfs/xfs_trans_inode.c             |   6 +-
 fs/xfs/xfs_iops.c                           |   6 +-
 fs/xfs/xfs_super.c                          |   2 +-
 include/linux/fs.h                          |  61 +++++---
 include/trace/events/timestamp.h            | 173 ++++++++++++++++++++++
 mm/shmem.c                                  |   2 +-
 13 files changed, 639 insertions(+), 79 deletions(-)
---
base-commit: 2e8c78ef85682671dae2ac3a5aa039b07be0fc0b
change-id: 20240626-mgtime-5cd80b18d810

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

[PATCH v2 01/11] fs: turn inode ctime fields into a single ktime_t

[Jeff Layton]

The ctime is not settable to arbitrary values. It always comes from the
system clock, so we'll never stamp an inode with a value that can't be
represented there. If we disregard people setting their system clock
past the year 2262, there is no reason we can't replace the ctime fields
with a ktime_t.

Switch the ctime fields to a single ktime_t. Move the i_generation down
above i_fsnotify_mask and then move the i_version into the resulting 8
byte hole. This shrinks struct inode by 8 bytes total, and should
improve the cache footprint as the i_version and ctime are usually
updated together.

The one downside I can see to switching to a ktime_t is that if someone
has a filesystem with files on it that has ctimes outside the ktime_t
range (before ~1678 AD or after ~2262 AD), we won't be able to display
them properly in stat() without some special treatment in the
filesystem. The operating assumption here is that that is not a
practical problem.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 include/linux/fs.h | 26 +++++++++++---------------
 1 file changed, 11 insertions(+), 15 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 2fa06a4d197a..7110d6dc9aab 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -662,11 +662,10 @@ struct inode {
 	loff_t			i_size;
 	time64_t		i_atime_sec;
 	time64_t		i_mtime_sec;
-	time64_t		i_ctime_sec;
 	u32			i_atime_nsec;
 	u32			i_mtime_nsec;
-	u32			i_ctime_nsec;
-	u32			i_generation;
+	ktime_t			__i_ctime;
+	atomic64_t		i_version;
 	spinlock_t		i_lock;	/* i_blocks, i_bytes, maybe i_size */
 	unsigned short          i_bytes;
 	u8			i_blkbits;
@@ -701,7 +700,6 @@ struct inode {
 		struct hlist_head	i_dentry;
 		struct rcu_head		i_rcu;
 	};
-	atomic64_t		i_version;
 	atomic64_t		i_sequence; /* see futex */
 	atomic_t		i_count;
 	atomic_t		i_dio_count;
@@ -724,6 +722,8 @@ struct inode {
 	};
 
 
+	u32			i_generation;
+
 #ifdef CONFIG_FSNOTIFY
 	__u32			i_fsnotify_mask; /* all events this inode cares about */
 	/* 32-bit hole reserved for expanding i_fsnotify_mask */
@@ -1608,29 +1608,25 @@ static inline struct timespec64 inode_set_mtime(struct inode *inode,
 	return inode_set_mtime_to_ts(inode, ts);
 }
 
-static inline time64_t inode_get_ctime_sec(const struct inode *inode)
+static inline struct timespec64 inode_get_ctime(const struct inode *inode)
 {
-	return inode->i_ctime_sec;
+	return ktime_to_timespec64(inode->__i_ctime);
 }
 
-static inline long inode_get_ctime_nsec(const struct inode *inode)
+static inline time64_t inode_get_ctime_sec(const struct inode *inode)
 {
-	return inode->i_ctime_nsec;
+	return inode_get_ctime(inode).tv_sec;
 }
 
-static inline struct timespec64 inode_get_ctime(const struct inode *inode)
+static inline long inode_get_ctime_nsec(const struct inode *inode)
 {
-	struct timespec64 ts = { .tv_sec  = inode_get_ctime_sec(inode),
-				 .tv_nsec = inode_get_ctime_nsec(inode) };
-
-	return ts;
+	return inode_get_ctime(inode).tv_nsec;
 }
 
 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;
+	inode->__i_ctime = ktime_set(ts.tv_sec, ts.tv_nsec);
 	return ts;
 }
 

-- 
2.45.2

[PATCH v2 02/11] fs: uninline inode_get_ctime and inode_set_ctime_to_ts

[Jeff Layton]

Move both functions to fs/inode.c as they have grown a little large for
inlining.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/inode.c         | 25 +++++++++++++++++++++++++
 include/linux/fs.h | 13 ++-----------
 2 files changed, 27 insertions(+), 11 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index e0815acc5abb..7b0a73ed499d 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2501,6 +2501,31 @@ struct timespec64 current_time(struct inode *inode)
 }
 EXPORT_SYMBOL(current_time);
 
+/**
+ * inode_get_ctime - fetch the current ctime from the inode
+ * @inode: inode from which to fetch ctime
+ *
+ * Grab the current ctime tv_nsec field from the inode, mask off the
+ * I_CTIME_QUERIED flag and return it. This is mostly intended for use by
+ * internal consumers of the ctime that aren't concerned with ensuring a
+ * fine-grained update on the next change (e.g. when preparing to store
+ * the value in the backing store for later retrieval).
+ */
+struct timespec64 inode_get_ctime(const struct inode *inode)
+{
+	ktime_t ctime = inode->__i_ctime;
+
+	return ktime_to_timespec64(ctime);
+}
+EXPORT_SYMBOL(inode_get_ctime);
+
+struct timespec64 inode_set_ctime_to_ts(struct inode *inode, struct timespec64 ts)
+{
+	inode->__i_ctime = ktime_set(ts.tv_sec, ts.tv_nsec);
+	return ts;
+}
+EXPORT_SYMBOL(inode_set_ctime_to_ts);
+
 /**
  * inode_set_ctime_current - set the ctime to current_time
  * @inode: inode
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 7110d6dc9aab..8e271c9e4a00 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1608,10 +1608,8 @@ static inline struct timespec64 inode_set_mtime(struct inode *inode,
 	return inode_set_mtime_to_ts(inode, ts);
 }
 
-static inline struct timespec64 inode_get_ctime(const struct inode *inode)
-{
-	return ktime_to_timespec64(inode->__i_ctime);
-}
+struct timespec64 inode_get_ctime(const struct inode *inode);
+struct timespec64 inode_set_ctime_to_ts(struct inode *inode, struct timespec64 ts);
 
 static inline time64_t inode_get_ctime_sec(const struct inode *inode)
 {
@@ -1623,13 +1621,6 @@ static inline long inode_get_ctime_nsec(const struct inode *inode)
 	return inode_get_ctime(inode).tv_nsec;
 }
 
-static inline struct timespec64 inode_set_ctime_to_ts(struct inode *inode,
-						      struct timespec64 ts)
-{
-	inode->__i_ctime = ktime_set(ts.tv_sec, ts.tv_nsec);
-	return ts;
-}
-
 /**
  * inode_set_ctime - set the ctime in the inode
  * @inode: inode in which to set the ctime

-- 
2.45.2

[PATCH v2 03/11] fs: tracepoints for inode_needs_update_time and inode_set_ctime_to_ts

[Jeff Layton]

Add a new tracepoint for when we're testing whether the timestamps need
updating, and around the update itself.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/inode.c                       |  4 +++
 include/trace/events/timestamp.h | 76 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 80 insertions(+)

diff --git a/fs/inode.c b/fs/inode.c
index 7b0a73ed499d..5d2b0dfe48c3 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -22,6 +22,8 @@
 #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"
 
 /*
@@ -2096,6 +2098,7 @@ static int inode_needs_update_time(struct inode *inode)
 	if (IS_I_VERSION(inode) && inode_iversion_need_inc(inode))
 		sync_it |= S_VERSION;
 
+	trace_inode_needs_update_time(inode, &now, &ts, sync_it);
 	return sync_it;
 }
 
@@ -2522,6 +2525,7 @@ EXPORT_SYMBOL(inode_get_ctime);
 struct timespec64 inode_set_ctime_to_ts(struct inode *inode, struct timespec64 ts)
 {
 	inode->__i_ctime = ktime_set(ts.tv_sec, ts.tv_nsec);
+	trace_inode_set_ctime_to_ts(inode, &ts);
 	return ts;
 }
 EXPORT_SYMBOL(inode_set_ctime_to_ts);
diff --git a/include/trace/events/timestamp.h b/include/trace/events/timestamp.h
new file mode 100644
index 000000000000..35ff875d3800
--- /dev/null
+++ b/include/trace/events/timestamp.h
@@ -0,0 +1,76 @@
+/* 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>
+
+TRACE_EVENT(inode_needs_update_time,
+	TP_PROTO(struct inode *inode,
+		 struct timespec64 *now,
+		 struct timespec64 *ctime,
+		 int sync_it),
+
+	TP_ARGS(inode, now, ctime, sync_it),
+
+	TP_STRUCT__entry(
+		__field(dev_t,			dev)
+		__field(ino_t,			ino)
+		__field(time64_t,		now_sec)
+		__field(time64_t,		ctime_sec)
+		__field(long,			now_nsec)
+		__field(long,			ctime_nsec)
+		__field(int,			sync_it)
+	),
+
+	TP_fast_assign(
+		__entry->dev		= inode->i_sb->s_dev;
+		__entry->ino		= inode->i_ino;
+		__entry->sync_it	= sync_it;
+		__entry->now_sec	= now->tv_sec;
+		__entry->ctime_sec	= ctime->tv_sec;
+		__entry->now_nsec	= now->tv_nsec;
+		__entry->ctime_nsec	= ctime->tv_nsec;
+		__entry->sync_it	= sync_it;
+	),
+
+	TP_printk("ino=%d:%d:%ld sync_it=%d now=%llu.%ld ctime=%llu.%lu",
+		MAJOR(__entry->dev), MINOR(__entry->dev), __entry->ino,
+		__entry->sync_it,
+		__entry->now_sec, __entry->now_nsec,
+		__entry->ctime_sec, __entry->ctime_nsec
+	)
+);
+
+TRACE_EVENT(inode_set_ctime_to_ts,
+	TP_PROTO(struct inode *inode,
+		 struct timespec64 *ts),
+
+	TP_ARGS(inode, ts),
+
+	TP_STRUCT__entry(
+		__field(dev_t,			dev)
+		__field(ino_t,			ino)
+		__field(time64_t,		ts_sec)
+		__field(long,			ts_nsec)
+	),
+
+	TP_fast_assign(
+		__entry->dev		= inode->i_sb->s_dev;
+		__entry->ino		= inode->i_ino;
+		__entry->ts_sec		= ts->tv_sec;
+		__entry->ts_nsec	= ts->tv_nsec;
+	),
+
+	TP_printk("ino=%d:%d:%ld ts=%llu.%lu",
+		MAJOR(__entry->dev), MINOR(__entry->dev), __entry->ino,
+		__entry->ts_sec, __entry->ts_nsec
+	)
+);
+#endif /* _TRACE_TIMESTAMP_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>

-- 
2.45.2

[PATCH v2 04/11] fs: add infrastructure for multigrain timestamps

[Jeff Layton]

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

What we need is a way to only use fine-grained timestamps when they are
being actively queried. Now that the ctime is stored as a ktime_t, we
can sacrifice the lowest bit in the word to act as a flag marking
whether the current timestamp has been queried via stat() or the like.

This solves the problem of being able to distinguish the timestamp
between updates, but introduces a new problem: it's now possible for a
file being changed to get a fine-grained timestamp and then a file that
was altered later to get a coarse-grained one that appears older than
the earlier fine-grained time. To remedy this, keep a global ktime_t
value that acts as a timestamp floor.

When we go to stamp a file, we first get the latter of the current floor
value and the current coarse-grained time (call this "now"). If the
current inode ctime hasn't been queried then we just attempt to stamp it
with that value using a cmpxchg() operation.

If it has been queried, then first see whether the current coarse time
appears later than what we have. If it does, then we accept that value.
If it doesn't, then we get a fine-grained time and try to swap that into
the global floor. Whether that succeeds or fails, we take the resulting
floor time and try to swap that into the ctime.

There is still one remaining problem:

All of this works as long as the realtime clock is monotonically
increasing. If the clock ever jumps backwards, then we could end up in a
situation where the floor value is "stuck" far in advance of the clock.

To remedy this, sanity check the floor value and if it's more than 6ms
(~2 jiffies) ahead of the current coarse-grained clock, disregard the
floor value, and just accept the current coarse-grained clock.

Filesystems opt into this by setting the FS_MGTIME fstype flag.  One
caveat: those that do will always present ctimes that have the lowest
bit unset, even when the on-disk ctime has it set.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/inode.c                       | 168 +++++++++++++++++++++++++++++++++------
 fs/stat.c                        |  39 ++++++++-
 include/linux/fs.h               |  30 +++++++
 include/trace/events/timestamp.h |  97 ++++++++++++++++++++++
 4 files changed, 306 insertions(+), 28 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 5d2b0dfe48c3..12790a26102c 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -62,6 +62,8 @@ static unsigned int i_hash_shift __ro_after_init;
 static struct hlist_head *inode_hashtable __ro_after_init;
 static __cacheline_aligned_in_smp DEFINE_SPINLOCK(inode_hash_lock);
 
+/* Don't send out a ctime lower than this (modulo backward clock jumps). */
+static __cacheline_aligned_in_smp ktime_t ctime_floor;
 /*
  * Empty aops. Can be used for the cases where the user does not
  * define any of the address_space operations.
@@ -2077,19 +2079,86 @@ int file_remove_privs(struct file *file)
 }
 EXPORT_SYMBOL(file_remove_privs);
 
+/*
+ * The coarse-grained clock ticks once per jiffy (every 2ms or so). If the
+ * current floor is >6ms in the future, assume that the clock has jumped
+ * backward.
+ */
+#define CTIME_FLOOR_MAX_NS	6000000
+
+/**
+ * coarse_ctime - return the current coarse-grained time
+ * @floor: current ctime_floor value
+ *
+ * Get the coarse-grained time, and then determine whether to
+ * return it or the current floor value. Returns the later of the
+ * floor and coarse grained time, unless the floor value is too
+ * far into the future. If that happens, assume the clock has jumped
+ * backward, and that the floor should be ignored.
+ */
+static ktime_t coarse_ctime(ktime_t floor)
+{
+	ktime_t now = ktime_get_coarse_real() & ~I_CTIME_QUERIED;
+
+	/* If coarse time is already newer, return that */
+	if (ktime_before(floor, now))
+		return now;
+
+	/* Ensure floor is not _too_ far in the future */
+	if (ktime_after(floor, now + CTIME_FLOOR_MAX_NS))
+		return now;
+
+	return floor;
+}
+
+/**
+ * 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.
+ */
+struct timespec64 current_time(struct inode *inode)
+{
+	ktime_t ctime, floor = smp_load_acquire(&ctime_floor);
+	ktime_t now = coarse_ctime(floor);
+	struct timespec64 now_ts = ktime_to_timespec64(now);
+
+	if (!is_mgtime(inode))
+		goto out;
+
+	/* If nothing has queried it, then coarse time is fine */
+	ctime = smp_load_acquire(&inode->__i_ctime);
+	if (ctime & I_CTIME_QUERIED) {
+		/*
+		 * If there is no apparent change, then
+		 * get a fine-grained timestamp.
+		 */
+		if ((now | I_CTIME_QUERIED) == ctime) {
+			ktime_get_real_ts64(&now_ts);
+			now_ts.tv_nsec &= ~I_CTIME_QUERIED;
+		}
+	}
+out:
+	return timestamp_truncate(now_ts, 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))
@@ -2485,25 +2554,6 @@ struct timespec64 timestamp_truncate(struct timespec64 t, struct inode *inode)
 }
 EXPORT_SYMBOL(timestamp_truncate);
 
-/**
- * current_time - Return FS time
- * @inode: inode.
- *
- * Return the current time truncated to the time granularity supported by
- * the fs.
- *
- * Note that inode and inode->sb cannot be NULL.
- * Otherwise, the function warns and returns time without truncation.
- */
-struct timespec64 current_time(struct inode *inode)
-{
-	struct timespec64 now;
-
-	ktime_get_coarse_real_ts64(&now);
-	return timestamp_truncate(now, inode);
-}
-EXPORT_SYMBOL(current_time);
-
 /**
  * inode_get_ctime - fetch the current ctime from the inode
  * @inode: inode from which to fetch ctime
@@ -2518,12 +2568,18 @@ struct timespec64 inode_get_ctime(const struct inode *inode)
 {
 	ktime_t ctime = inode->__i_ctime;
 
+	if (is_mgtime(inode))
+		ctime &= ~I_CTIME_QUERIED;
 	return ktime_to_timespec64(ctime);
 }
 EXPORT_SYMBOL(inode_get_ctime);
 
 struct timespec64 inode_set_ctime_to_ts(struct inode *inode, struct timespec64 ts)
 {
+	trace_inode_set_ctime_to_ts(inode, &ts);
+
+	if (is_mgtime(inode))
+		ts.tv_nsec &= ~I_CTIME_QUERIED;
 	inode->__i_ctime = ktime_set(ts.tv_sec, ts.tv_nsec);
 	trace_inode_set_ctime_to_ts(inode, &ts);
 	return ts;
@@ -2535,14 +2591,74 @@ EXPORT_SYMBOL(inode_set_ctime_to_ts);
  * @inode: inode
  *
  * Set the inode->i_ctime to the current value for the inode. Returns
- * the current value that was assigned to i_ctime.
+ * the current value that was assigned to i_ctime. If this is a not
+ * multigrain inode, then we just set it to whatever the coarse time is.
+ *
+ * If it is multigrain, then we first see if the coarse-grained
+ * timestamp is distinct from what we have. If so, then we'll just use
+ * that. If we have to get a fine-grained timestamp, then do so, and
+ * try to swap it into the floor. We accept the new floor value
+ * regardless of the outcome of the cmpxchg. After that, we try to
+ * swap the new value into __i_ctime. Again, we take the resulting
+ * ctime, regardless of the outcome of the swap.
  */
 struct timespec64 inode_set_ctime_current(struct inode *inode)
 {
-	struct timespec64 now = current_time(inode);
+	ktime_t ctime, now, cur, floor = smp_load_acquire(&ctime_floor);
+
+	now = coarse_ctime(floor);
 
-	inode_set_ctime_to_ts(inode, now);
-	return now;
+	/* Just return that if this is not a multigrain fs */
+	if (!is_mgtime(inode)) {
+		inode->__i_ctime = now;
+		goto out;
+	}
+
+	/*
+	 * We only need a fine-grained time if someone has queried it,
+	 * and the current coarse grained time isn't later than what's
+	 * already there.
+	 */
+	ctime = smp_load_acquire(&inode->__i_ctime);
+	if ((ctime & I_CTIME_QUERIED) && !ktime_after(now, ctime & ~I_CTIME_QUERIED)) {
+		ktime_t old;
+
+		/* Get a fine-grained time */
+		now = ktime_get_real() & ~I_CTIME_QUERIED;
+
+		/*
+		 * If the cmpxchg works, we take the new floor value. If
+		 * not, then that means that someone else changed it after we
+		 * fetched it but before we got here. That value is just
+		 * as good, so keep it.
+		 */
+		old = cmpxchg(&ctime_floor, floor, now);
+		trace_ctime_floor_update(inode, floor, now, old);
+		if (old != floor)
+			now = old;
+	}
+retry:
+	/* Try to swap the ctime into place. */
+	cur = cmpxchg(&inode->__i_ctime, ctime, now);
+	trace_ctime_inode_update(inode, ctime, now, cur);
+
+	/* If swap occurred, then we're done */
+	if (cur != ctime) {
+		/*
+		 * 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 (!(ctime & I_CTIME_QUERIED) && (ctime | I_CTIME_QUERIED) == cur) {
+			ctime = cur;
+			goto retry;
+		}
+		/* Otherwise, take the new ctime */
+		now = cur & ~I_CTIME_QUERIED;
+	}
+out:
+	return timestamp_truncate(ktime_to_timespec64(now), inode);
 }
 EXPORT_SYMBOL(inode_set_ctime_current);
 
diff --git a/fs/stat.c b/fs/stat.c
index 6f65b3456cad..7e9bd16b553b 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -22,10 +22,39 @@
 
 #include <linux/uaccess.h>
 #include <asm/unistd.h>
+#include <trace/events/timestamp.h>
 
 #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 so the next write
+ * will ensure a distinct timestamp.
+ */
+void fill_mg_cmtime(struct kstat *stat, u32 request_mask, struct inode *inode)
+{
+	atomic_long_t *pc = (atomic_long_t *)&inode->__i_ctime;
+
+	/* 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.tv_sec = inode->i_mtime_sec;
+	stat->mtime.tv_nsec = inode->i_mtime_nsec;
+	stat->ctime = ktime_to_timespec64(atomic_long_fetch_or(I_CTIME_QUERIED, pc) &
+						~I_CTIME_QUERIED);
+	trace_fill_mg_cmtime(inode, atomic_long_read(pc));
+}
+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 +87,14 @@ 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 8e271c9e4a00..8601425ac249 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1608,6 +1608,23 @@ 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.
+ *
+ * We use the least significant bit of the ktime_t to track the QUERIED
+ * flag. This means that filesystems with multigrain timestamps effectively
+ * have 2ns resolution for the ctime, even if they advertise 1ns s_time_gran.
+ */
+#define I_CTIME_QUERIED		(1LL)
+
+static inline bool is_mgtime(const struct inode *inode);
+
 struct timespec64 inode_get_ctime(const struct inode *inode);
 struct timespec64 inode_set_ctime_to_ts(struct inode *inode, struct timespec64 ts);
 
@@ -2477,6 +2494,7 @@ 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;
@@ -2500,6 +2518,17 @@ 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));
@@ -3234,6 +3263,7 @@ 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);
 extern int vfs_getattr_nosec(const struct path *, struct kstat *, u32, unsigned int);
diff --git a/include/trace/events/timestamp.h b/include/trace/events/timestamp.h
index 35ff875d3800..1f71738aa38c 100644
--- a/include/trace/events/timestamp.h
+++ b/include/trace/events/timestamp.h
@@ -8,6 +8,78 @@
 #include <linux/tracepoint.h>
 #include <linux/fs.h>
 
+TRACE_EVENT(ctime_floor_update,
+	TP_PROTO(struct inode *inode,
+		 ktime_t old,
+		 ktime_t new,
+		 ktime_t cur),
+
+	TP_ARGS(inode, old, new, cur),
+
+	TP_STRUCT__entry(
+		__field(dev_t,				dev)
+		__field(ino_t,				ino)
+		__field(ktime_t,			old)
+		__field(ktime_t,			new)
+		__field(ktime_t,			cur)
+	),
+
+	TP_fast_assign(
+		__entry->dev		= inode->i_sb->s_dev;
+		__entry->ino		= inode->i_ino;
+		__entry->old		= old;
+		__entry->new		= new;
+		__entry->cur		= cur;
+	),
+
+	TP_printk("ino=%d:%d:%lu old=%llu.%lu new=%llu.%lu cur=%llu.%lu swp=%c",
+		MAJOR(__entry->dev), MINOR(__entry->dev), __entry->ino,
+		ktime_to_timespec64(__entry->old).tv_sec,
+		ktime_to_timespec64(__entry->old).tv_nsec,
+		ktime_to_timespec64(__entry->new).tv_sec,
+		ktime_to_timespec64(__entry->new).tv_nsec,
+		ktime_to_timespec64(__entry->cur).tv_sec,
+		ktime_to_timespec64(__entry->cur).tv_nsec,
+		(__entry->old == __entry->cur) ? 'Y' : 'N'
+	)
+);
+
+TRACE_EVENT(ctime_inode_update,
+	TP_PROTO(struct inode *inode,
+		 ktime_t old,
+		 ktime_t new,
+		 ktime_t cur),
+
+	TP_ARGS(inode, old, new, cur),
+
+	TP_STRUCT__entry(
+		__field(dev_t,				dev)
+		__field(ino_t,				ino)
+		__field(ktime_t,			old)
+		__field(ktime_t,			new)
+		__field(ktime_t,			cur)
+	),
+
+	TP_fast_assign(
+		__entry->dev		= inode->i_sb->s_dev;
+		__entry->ino		= inode->i_ino;
+		__entry->old		= old;
+		__entry->new		= new;
+		__entry->cur		= cur;
+	),
+
+	TP_printk("ino=%d:%d:%ld old=%llu.%ld new=%llu.%ld cur=%llu.%ld swp=%c",
+		MAJOR(__entry->dev), MINOR(__entry->dev), __entry->ino,
+		ktime_to_timespec64(__entry->old).tv_sec,
+		ktime_to_timespec64(__entry->old).tv_nsec,
+		ktime_to_timespec64(__entry->new).tv_sec,
+		ktime_to_timespec64(__entry->new).tv_nsec,
+		ktime_to_timespec64(__entry->cur).tv_sec,
+		ktime_to_timespec64(__entry->cur).tv_nsec,
+		(__entry->old == __entry->cur ? 'Y' : 'N')
+	)
+);
+
 TRACE_EVENT(inode_needs_update_time,
 	TP_PROTO(struct inode *inode,
 		 struct timespec64 *now,
@@ -70,6 +142,31 @@ TRACE_EVENT(inode_set_ctime_to_ts,
 		__entry->ts_sec, __entry->ts_nsec
 	)
 );
+
+TRACE_EVENT(fill_mg_cmtime,
+	TP_PROTO(struct inode *inode,
+		 ktime_t ctime),
+
+	TP_ARGS(inode, ctime),
+
+	TP_STRUCT__entry(
+		__field(dev_t,			dev)
+		__field(ino_t,			ino)
+		__field(ktime_t,		ctime)
+	),
+
+	TP_fast_assign(
+		__entry->dev		= inode->i_sb->s_dev;
+		__entry->ino		= inode->i_ino;
+		__entry->ctime		= ctime;
+	),
+
+	TP_printk("ino=%d:%d:%ld ctime=%llu.%lu",
+		MAJOR(__entry->dev), MINOR(__entry->dev), __entry->ino,
+		ktime_to_timespec64(__entry->ctime).tv_sec,
+		ktime_to_timespec64(__entry->ctime).tv_nsec
+	)
+);
 #endif /* _TRACE_TIMESTAMP_H */
 
 /* This part must be outside protection */

-- 
2.45.2

[PATCH v2 05/11] fs: add percpu counters to count fine vs. coarse timestamps

[Jeff Layton]

Keep a pair of percpu counters so we can track what proportion of
timestamps is fine-grained.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/inode.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/fs/inode.c b/fs/inode.c
index 12790a26102c..5b5a1a8c0bb7 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>
@@ -64,6 +66,10 @@ static __cacheline_aligned_in_smp DEFINE_SPINLOCK(inode_hash_lock);
 
 /* Don't send out a ctime lower than this (modulo backward clock jumps). */
 static __cacheline_aligned_in_smp ktime_t ctime_floor;
+
+static struct percpu_counter mg_fine_ts;
+static struct percpu_counter mg_coarse_ts;
+
 /*
  * Empty aops. Can be used for the cases where the user does not
  * define any of the address_space operations.
@@ -2636,6 +2642,9 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
 		trace_ctime_floor_update(inode, floor, now, old);
 		if (old != floor)
 			now = old;
+		percpu_counter_inc(&mg_fine_ts);
+	} else {
+		percpu_counter_inc(&mg_coarse_ts);
 	}
 retry:
 	/* Try to swap the ctime into place. */
@@ -2711,3 +2720,32 @@ umode_t mode_strip_sgid(struct mnt_idmap *idmap,
 	return mode & ~S_ISGID;
 }
 EXPORT_SYMBOL(mode_strip_sgid);
+
+static int mgts_show(struct seq_file *s, void *p)
+{
+	u64 fine = percpu_counter_sum(&mg_fine_ts);
+	u64 coarse = percpu_counter_sum(&mg_coarse_ts);
+
+	seq_printf(s, "%llu %llu\n", fine, coarse);
+	return 0;
+}
+
+DEFINE_SHOW_ATTRIBUTE(mgts);
+
+static int __init mg_debugfs_init(void)
+{
+	int ret = percpu_counter_init(&mg_fine_ts, 0, GFP_KERNEL);
+
+	if (ret)
+		return ret;
+
+	ret = percpu_counter_init(&mg_coarse_ts, 0, GFP_KERNEL);
+	if (ret) {
+		percpu_counter_destroy(&mg_fine_ts);
+		return ret;
+	}
+
+	debugfs_create_file("multigrain_timestamps", S_IFREG | S_IRUGO, NULL, NULL, &mgts_fops);
+	return 0;
+}
+late_initcall(mg_debugfs_init);

-- 
2.45.2

[PATCH v2 06/11] 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, we need to 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().

Note that we don't want to do this universally however, as some
filesystems (e.g. most networked fs) want to do an explicit update
elsewhere before updating the local inode.

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 825007d5cda4..e03ea6951864 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, we need to 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 we're setting those 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);
 

-- 
2.45.2

[PATCH v2 07/11] 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.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/xfs/libxfs/xfs_trans_inode.c | 6 +++---
 fs/xfs/xfs_iops.c               | 6 ++++--
 fs/xfs/xfs_super.c              | 2 +-
 3 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_trans_inode.c b/fs/xfs/libxfs/xfs_trans_inode.c
index 69fc5b981352..1f3639bbf5f0 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_CREATE)
 		ip->i_crtime = tv;
 }
diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index ff222827e550..ed6e6d9507df 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -590,10 +590,12 @@ 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)) {
 		if (request_mask & STATX_BTIME) {
 			stat->result_mask |= STATX_BTIME;
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 27e9f749c4c7..210481b03fdb 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");
 

-- 
2.45.2

[PATCH v2 08/11] 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.

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 c682fb927b64..9ae48763f81f 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -7310,7 +7310,7 @@ 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");
 

-- 
2.45.2

[PATCH v2 09/11] 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.

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 d90138683a0a..409628c0c3cc 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);
-}
-
 static int btrfs_write_check(struct kiocb *iocb, struct iov_iter *from,
 			     size_t count)
 {
@@ -1171,7 +1151,10 @@ static int btrfs_write_check(struct kiocb *iocb, struct iov_iter *from,
 	 * 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 f05cce7c8b8d..1cd50293b98d 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2173,7 +2173,8 @@ 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");

-- 
2.45.2

[PATCH v2 10/11] 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.

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 8cdd27db042b..60a8e05eed34 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -4653,7 +4653,7 @@ 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)

-- 
2.45.2

[PATCH v2 11/11] 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.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 Documentation/filesystems/multigrain-ts.rst | 126 ++++++++++++++++++++++++++++
 1 file changed, 126 insertions(+)

diff --git a/Documentation/filesystems/multigrain-ts.rst b/Documentation/filesystems/multigrain-ts.rst
new file mode 100644
index 000000000000..beef7f79108c
--- /dev/null
+++ b/Documentation/filesystems/multigrain-ts.rst
@@ -0,0 +1,126 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================
+Multigrain Timestamps
+=====================
+
+Introduction
+============
+Historically, the kernel has always used a coarse time values to stamp
+inodes. This value is updated on 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, and many just populating this with
+the ctime.
+
+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 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.
+
+Multigrain Timestamps
+=====================
+Multigrain timestamps are aimed at ensuring that changes to a single file are
+always recognizeable, 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 the lowest-order bit in the timestamp as a flag that indicates whether
+the mtime or ctime have 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, we maintain a per-time_namespace 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.
+
+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 you should have it call fill_mg_cmtime to
+fill those values.
+
+Caveats
+=======
+The main sacrifice is the lowest bit in the ctime's field, since that's
+where the flag is stored. Thus, timestamps presented by multigrain enabled
+filesystems will always have an even tv_nsec value (since the lowest bit
+is masked off).

-- 
2.45.2

Re: [PATCH v2 07/11] xfs: switch to multigrain timestamps

[Josef Bacik]

On Mon, Jul 01, 2024 at 06:26:43AM -0400, Jeff Layton wrote:
> 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.
> 
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>  fs/xfs/libxfs/xfs_trans_inode.c | 6 +++---
>  fs/xfs/xfs_iops.c               | 6 ++++--
>  fs/xfs/xfs_super.c              | 2 +-
>  3 files changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_trans_inode.c b/fs/xfs/libxfs/xfs_trans_inode.c
> index 69fc5b981352..1f3639bbf5f0 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_CREATE)
>  		ip->i_crtime = tv;
>  }
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index ff222827e550..ed6e6d9507df 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -590,10 +590,12 @@ 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);
>  
> +

Stray newline.  Thanks,

Josef

Re: [PATCH v2 09/11] btrfs: convert to multigrain timestamps

[Josef Bacik]

On Mon, Jul 01, 2024 at 06:26:45AM -0400, Jeff Layton wrote:
> 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.
> 
> 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 d90138683a0a..409628c0c3cc 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);
> -}
> -
>  static int btrfs_write_check(struct kiocb *iocb, struct iov_iter *from,
>  			     size_t count)
>  {
> @@ -1171,7 +1151,10 @@ static int btrfs_write_check(struct kiocb *iocb, struct iov_iter *from,
>  	 * 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);

You've dropped the

if (IS_I_VERSION(inode))

check here, and it doesn't appear to be in inode_inc_iversion.  Is there a
reason for this?  Thanks,

Josef

Re: [PATCH v2 11/11] Documentation: add a new file documenting multigrain timestamps

[Josef Bacik]

On Mon, Jul 01, 2024 at 06:26:47AM -0400, Jeff Layton wrote:
> Add a high-level document that describes how multigrain timestamps work,
> rationale for them, and some info about implementation and tradeoffs.
> 
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>  Documentation/filesystems/multigrain-ts.rst | 126 ++++++++++++++++++++++++++++
>  1 file changed, 126 insertions(+)
> 
> diff --git a/Documentation/filesystems/multigrain-ts.rst b/Documentation/filesystems/multigrain-ts.rst
> new file mode 100644
> index 000000000000..beef7f79108c
> --- /dev/null
> +++ b/Documentation/filesystems/multigrain-ts.rst
> @@ -0,0 +1,126 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +=====================
> +Multigrain Timestamps
> +=====================
> +
> +Introduction
> +============
> +Historically, the kernel has always used a coarse time values to stamp
> +inodes. This value is updated on 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, and many just populating this with
> +the ctime.
> +
> +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 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.
> +
> +Multigrain Timestamps
> +=====================
> +Multigrain timestamps are aimed at ensuring that changes to a single file are
> +always recognizeable, 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 the lowest-order bit in the timestamp as a flag that indicates whether
> +the mtime or ctime have 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, we maintain a per-time_namespace floor value that

You dropped this bit in the series, so this isn't correct, should just be

"we maintain a floor value"

Thanks,

Josef

Re: [PATCH v2 00/11] fs: multigrain timestamp redux

[Josef Bacik]

On Mon, Jul 01, 2024 at 06:26:36AM -0400, Jeff Layton wrote:
> This set is essentially unchanged from the last one, aside from the
> new file in Documentation/. I had a review comment from Andi Kleen
> suggesting that the ctime_floor should be per time_namespace, but I
> think that's incorrect as the realtime clock is not namespaced.
> 
> At LSF/MM this year, we had a discussion about the inode change
> attribute. At the time I mentioned that I thought I could salvage the
> multigrain timestamp work that had to be reverted last year [1].  That
> version had to be reverted because it was possible for a file to get a
> coarse grained timestamp that appeared to be earlier than another file
> that had recently gotten a fine-grained stamp.
> 
> This version corrects the problem by establishing a per-time_namespace
> ctime_floor value that should prevent this from occurring. In the above
> situation that was problematic before, the two files might end up with
> the same timestamp value, but they won't appear to have been modified in
> the wrong order.
> 
> That problem was discovered by the test-stat-time gnulib test. Note that
> that test still fails on multigrain timestamps, but that's because its
> method of determining the minimum delay that will show a timestamp
> change will no longer work with multigrain timestamps. I have a patch to
> change the testcase to use a different method that I've posted to the
> bug-gnulib mailing list.
> 
> The big question with this set is whether the performance will be
> suitable. The testing I've done seems to show performance parity with
> multigrain timestamps enabled, but it's hard to rule this out regressing
> some workload.
> 
> This set is based on top of Christian's vfs.misc branch (which has the
> earlier change to track inode timestamps as discrete integers). If there
> are no major objections, I'd like to let this soak in linux-next for a
> bit to see if any problems shake out.
> 
> [1]: https://lore.kernel.org/linux-fsdevel/20230807-mgctime-v7-0-d1dec143a704@kernel.org/
> 
> Signed-off-by: Jeff Layton <jlayton@kernel.org>

I have a few nits that need to be addressed, but you can add

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

to the series once they're addressed.  Thanks,

Josef

Re: [PATCH v2 09/11] btrfs: convert to multigrain timestamps

[Jeff Layton]

On Mon, 2024-07-01 at 09:49 -0400, Josef Bacik wrote:
> On Mon, Jul 01, 2024 at 06:26:45AM -0400, Jeff Layton wrote:
> > 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.
> > 
> > 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 d90138683a0a..409628c0c3cc 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);
> > -}
> > -
> >  static int btrfs_write_check(struct kiocb *iocb, struct iov_iter
> > *from,
> >  			     size_t count)
> >  {
> > @@ -1171,7 +1151,10 @@ static int btrfs_write_check(struct kiocb
> > *iocb, struct iov_iter *from,
> >  	 * 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);
> 
> You've dropped the
> 
> if (IS_I_VERSION(inode))
> 
> check here, and it doesn't appear to be in inode_inc_iversion.  Is
> there a
> reason for this?  Thanks,
> 

AFAICT, btrfs always sets SB_I_VERSION. Are there any cases where it
isn't? If so, then I can put this check back. I'll make a note about it
in the changelog if not.

-- 
Jeff Layton <jlayton@kernel.org>

Re: [PATCH v2 00/11] fs: multigrain timestamp redux

[Jeff Layton]

On Mon, 2024-07-01 at 09:53 -0400, Josef Bacik wrote:
> On Mon, Jul 01, 2024 at 06:26:36AM -0400, Jeff Layton wrote:
> > This set is essentially unchanged from the last one, aside from the
> > new file in Documentation/. I had a review comment from Andi Kleen
> > suggesting that the ctime_floor should be per time_namespace, but I
> > think that's incorrect as the realtime clock is not namespaced.
> > 
> > At LSF/MM this year, we had a discussion about the inode change
> > attribute. At the time I mentioned that I thought I could salvage the
> > multigrain timestamp work that had to be reverted last year [1].  That
> > version had to be reverted because it was possible for a file to get a
> > coarse grained timestamp that appeared to be earlier than another file
> > that had recently gotten a fine-grained stamp.
> > 
> > This version corrects the problem by establishing a per-time_namespace
> > ctime_floor value that should prevent this from occurring. In the above
> > situation that was problematic before, the two files might end up with
> > the same timestamp value, but they won't appear to have been modified in
> > the wrong order.
> > 
> > That problem was discovered by the test-stat-time gnulib test. Note that
> > that test still fails on multigrain timestamps, but that's because its
> > method of determining the minimum delay that will show a timestamp
> > change will no longer work with multigrain timestamps. I have a patch to
> > change the testcase to use a different method that I've posted to the
> > bug-gnulib mailing list.
> > 
> > The big question with this set is whether the performance will be
> > suitable. The testing I've done seems to show performance parity with
> > multigrain timestamps enabled, but it's hard to rule this out regressing
> > some workload.
> > 
> > This set is based on top of Christian's vfs.misc branch (which has the
> > earlier change to track inode timestamps as discrete integers). If there
> > are no major objections, I'd like to let this soak in linux-next for a
> > bit to see if any problems shake out.
> > 
> > [1]: https://lore.kernel.org/linux-fsdevel/20230807-mgctime-v7-0-d1dec143a704@kernel.org/
> > 
> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> 
> I have a few nits that need to be addressed, but you can add
> 
> Reviewed-by: Josef Bacik <josef@toxicpanda.com>
> 
> to the series once they're addressed.  Thanks,
> 

Thanks! Fixed them up in my tree. I left the IS_I_VERSION check out as
well, and added a note to the changelog on the btrfs patch.
-- 
Jeff Layton <jlayton@kernel.org>

Re: [PATCH v2 09/11] btrfs: convert to multigrain timestamps

[David Sterba]

On Mon, Jul 01, 2024 at 09:57:43AM -0400, Jeff Layton wrote:
> On Mon, 2024-07-01 at 09:49 -0400, Josef Bacik wrote:
> > On Mon, Jul 01, 2024 at 06:26:45AM -0400, Jeff Layton wrote:
> > > 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.
> > > 
> > > 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 d90138683a0a..409628c0c3cc 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);
> > > -}
> > > -
> > >  static int btrfs_write_check(struct kiocb *iocb, struct iov_iter
> > > *from,
> > >  			     size_t count)
> > >  {
> > > @@ -1171,7 +1151,10 @@ static int btrfs_write_check(struct kiocb
> > > *iocb, struct iov_iter *from,
> > >  	 * 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);
> > 
> > You've dropped the
> > 
> > if (IS_I_VERSION(inode))
> > 
> > check here, and it doesn't appear to be in inode_inc_iversion.  Is
> > there a
> > reason for this?  Thanks,
> > 
> 
> AFAICT, btrfs always sets SB_I_VERSION. Are there any cases where it
> isn't? If so, then I can put this check back. I'll make a note about it
> in the changelog if not.

Yes it's always set and I don't see anything in the generic code that
would unset it so it's safe to drop the IS_I_VERSION check.

The check was originally added in November 2012 by 6c760c072403f4
("Btrfs: do not call file_update_time in aio_write") and then moved a
few times. Enabling the super block flags was added in May 2012 by
0c4d2d95d06e92 ("Btrfs: use i_version instead of our own sequence") so
the check was not necessary from the beginning.

Re: [PATCH v2 09/11] btrfs: convert to multigrain timestamps

[Josef Bacik]

On Mon, Jul 01, 2024 at 09:57:43AM -0400, Jeff Layton wrote:
> On Mon, 2024-07-01 at 09:49 -0400, Josef Bacik wrote:
> > On Mon, Jul 01, 2024 at 06:26:45AM -0400, Jeff Layton wrote:
> > > 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.
> > > 
> > > 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 d90138683a0a..409628c0c3cc 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);
> > > -}
> > > -
> > >  static int btrfs_write_check(struct kiocb *iocb, struct iov_iter
> > > *from,
> > >  			     size_t count)
> > >  {
> > > @@ -1171,7 +1151,10 @@ static int btrfs_write_check(struct kiocb
> > > *iocb, struct iov_iter *from,
> > >  	 * 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);
> > 
> > You've dropped the
> > 
> > if (IS_I_VERSION(inode))
> > 
> > check here, and it doesn't appear to be in inode_inc_iversion.  Is
> > there a
> > reason for this?  Thanks,
> > 
> 
> AFAICT, btrfs always sets SB_I_VERSION. Are there any cases where it
> isn't? If so, then I can put this check back. I'll make a note about it
> in the changelog if not.

Ah ok I'm dumb, ignore me, thanks,

Josef