[PATCH 19/19] fs: handle inode->i_version more efficiently

[Jeff Layton]

From: Jeff Layton <jlayton@xxxxxxxxxx>

Since i_version is mostly treated as an opaque value, we can exploit that
fact to avoid incrementing it when no one is watching. With that change,
we can avoid incrementing the counter on writes, unless someone has
queried for it since it was last incremented. If the a/c/mtime don't
change, and the i_version hasn't changed, then there's no need to dirty
the inode metadata on a write.

Convert the i_version counter to an atomic64_t, and use the lowest order
bit to hold a flag that will tell whether anyone has queried the value
since it was last incremented.

When we go to maybe increment it, we fetch the value and check the flag
bit.  If it's clear then we don't need to do anything if the update
isn't being forced.

If we do need to update, then we increment the counter by 2, and clear
the flag bit, and then use a CAS op to swap it into place. If that
works, we return true. If it doesn't then do it again with the value
that we fetch from the CAS operation.

On the query side, if the flag is already set, then we just shift the
value down by 1 bit and return it. Otherwise, we set the flag in our
on-stack value and again use cmpxchg to swap it into place if it hasn't
changed. If it has, then we use the value from the cmpxchg as the new
"old" value and try again.

This method allows us to avoid incrementing the counter on writes (and
dirtying the metadata) under typical workloads. We only need to increment
if it has been queried since it was last changed.

Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
---
 include/linux/fs.h | 150 +++++++++++++++++++++++++++++++++++------------------
 1 file changed, 99 insertions(+), 51 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index c234fac4bb77..84fe3ce8e45a 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -639,7 +639,7 @@ struct inode {
 		struct hlist_head	i_dentry;
 		struct rcu_head		i_rcu;
 	};
-	u64			i_version;
+	atomic64_t		i_version;
 	atomic_t		i_count;
 	atomic_t		i_dio_count;
 	atomic_t		i_writecount;
@@ -2059,86 +2059,116 @@ static inline void inode_dec_link_count(struct inode *inode)
  * i_version on namespace changes in directories (mkdir, rmdir, unlink, etc.).
  * We consider these sorts of filesystems to have a kernel-managed i_version.
  *
- * Note that some filesystems (e.g. NFS and AFS) just use the field to store
- * a server-provided value (for the most part). For that reason, those
- * filesystems do not set SB_I_VERSION. These filesystems are considered to
- * have a self-managed i_version.
+ * This implementation uses the low bit in the i_version field as a flag to
+ * track when the value has been queried. If it has not been queried since it
+ * was last incremented, we can skip the increment in most cases.
+ *
+ * In the event that we're updating the ctime, we will usually go ahead and
+ * bump the i_version anyway. Since that has to go to stable storage in some
+ * fashion, we might as well increment it as well.
+ */
+
+/*
+ * We borrow the lowest bit in the i_version to use as a flag to tell whether
+ * it has been queried since we last incremented it. If it has, then we must
+ * increment it on the next change. After that, we can clear the flag and
+ * avoid incrementing it again until it has again been queried.
  */
+#define I_VERSION_QUERIED_SHIFT	(1)
+#define I_VERSION_QUERIED	(1ULL << (I_VERSION_QUERIED_SHIFT - 1))
+#define I_VERSION_INCREMENT	(1ULL << I_VERSION_QUERIED_SHIFT)
 
 /**
  * inode_set_iversion_raw - set i_version to the specified raw value
  * @inode: inode to set
- * @new: new i_version value to set
+ * @val: new i_version value to set
  *
- * Set @inode's i_version field to @new. This function is for use by
+ * Set @inode's i_version field to @val. This function is for use by
  * filesystems that self-manage the i_version.
  *
  * For example, the NFS client stores its NFSv4 change attribute in this way,
  * and the AFS client stores the data_version from the server here.
  */
 static inline void
-inode_set_iversion_raw(struct inode *inode, const u64 new)
+inode_set_iversion_raw(struct inode *inode, const u64 val)
 {
-	inode->i_version = new;
+	atomic64_set(&inode->i_version, val);
 }
 
 /**
  * inode_set_iversion - set i_version to a particular value
  * @inode: inode to set
- * @new: new i_version value to set
+ * @val: new i_version value to set
  *
- * Set @inode's i_version field to @new. This function is for filesystems with
- * a kernel-managed i_version.
+ * Set @inode's i_version field to @val. This function is for filesystems with
+ * a kernel-managed i_version, for initializing a newly-created inode from
+ * scratch.
  *
- * For now, this just does the same thing as the _raw variant.
+ * In this case, we do not set the QUERIED flag since we know that this value
+ * has never been queried.
  */
 static inline void
-inode_set_iversion(struct inode *inode, const u64 new)
+inode_set_iversion(struct inode *inode, const u64 val)
 {
-	inode_set_iversion_raw(inode, new);
+	inode_set_iversion_raw(inode, val << I_VERSION_QUERIED_SHIFT);
 }
 
 /**
- * inode_set_iversion_queried - set i_version to a particular value and set
- *                              flag to indicate that it has been viewed
+ * inode_set_iversion_queried - set i_version to a particular value as quereied
  * @inode: inode to set
- * @new: new i_version value to set
+ * @val: new i_version value to set
+ *
+ * Set @inode's i_version field to @val, and flag it for increment on the next
+ * change.
  *
  * When loading in an i_version value from a backing store, we typically don't
- * know whether it was previously viewed before being stored or not. Thus, we
+ * know whether it was previously viewed before being stored. Thus, we
  * must assume that it was, to ensure that any changes will result in the
  * value changing.
- *
- * This function will set the inode's i_version, and possibly flag the value
- * as if it has already been viewed at least once.
- *
- * For now, this just does what inode_set_iversion does.
  */
 static inline void
-inode_set_iversion_queried(struct inode *inode, const u64 new)
+inode_set_iversion_queried(struct inode *inode, const u64 val)
 {
-	inode_set_iversion(inode, new);
+	inode_set_iversion_raw(inode, (val << I_VERSION_QUERIED_SHIFT) |
+				I_VERSION_QUERIED);
 }
 
 /**
  * inode_maybe_inc_iversion - increments i_version
  * @inode: inode with the i_version that should be updated
- * @force: increment the counter even if it's not necessary
+ * @force: increment the counter even if it's not necessary?
  *
  * Every time the inode is modified, the i_version field must be seen to have
  * changed by any observer.
  *
- * In this implementation, we always increment it after taking the i_lock to
- * ensure that we don't race with other incrementors.
+ * If "force" is set or the QUERIED flag is set, then ensure that we increment
+ * the value, and clear the queried flag.
+ *
+ * In the common case where neither is set, then we can return "false" without
+ * updating i_version.
  *
- * Returns true if counter was bumped, and false if it wasn't.
+ * If this function returns false, and no other metadata has changed, then we
+ * can avoid logging the metadata.
  */
 static inline bool
 inode_maybe_inc_iversion(struct inode *inode, bool force)
 {
-	atomic64_t *ivp = (atomic64_t *)&inode->i_version;
+	u64 cur, old, new;
+
+	cur = (u64)atomic64_read(&inode->i_version);
+	for (;;) {
+		/* If flag is clear then we needn't do anything */
+		if (!force && !(cur & I_VERSION_QUERIED))
+			return false;
 
-	atomic64_inc(ivp);
+		/* Since lowest bit is flag, add 2 to avoid it */
+		new = (cur & ~I_VERSION_QUERIED) + I_VERSION_INCREMENT;
+
+		old = atomic64_cmpxchg(&inode->i_version, cur, new);
+		if (likely(old == cur))
+			break;
+		cur = old;
+	}
 	return true;
 }
 
@@ -2155,21 +2185,6 @@ inode_inc_iversion(struct inode *inode)
 	inode_maybe_inc_iversion(inode, true);
 }
 
-/**
- * inode_iversion_need_inc - is the i_version in need of being incremented?
- * @inode: inode to check
- *
- * Returns whether the inode->i_version counter needs incrementing on the next
- * change.
- *
- * For now, we assume that it always does.
- */
-static inline bool
-inode_iversion_need_inc(struct inode *inode)
-{
-	return true;
-}
-
 /**
  * inode_peek_iversion_raw - grab a "raw" iversion value
  * @inode: inode from which i_version should be read
@@ -2184,7 +2199,20 @@ inode_iversion_need_inc(struct inode *inode)
 static inline u64
 inode_peek_iversion_raw(const struct inode *inode)
 {
-	return inode->i_version;
+	return atomic64_read(&inode->i_version);
+}
+
+/**
+ * inode_iversion_need_inc - is the i_version in need of being incremented?
+ * @inode: inode to check
+ *
+ * Returns whether the inode->i_version counter needs incrementing on the next
+ * change. Just fetch the value and check the QUERIED flag.
+ */
+static inline bool
+inode_iversion_need_inc(struct inode *inode)
+{
+	return inode_peek_iversion_raw(inode) & I_VERSION_QUERIED;
 }
 
 /**
@@ -2201,7 +2229,7 @@ inode_peek_iversion_raw(const struct inode *inode)
 static inline u64
 inode_peek_iversion(const struct inode *inode)
 {
-	return inode_peek_iversion_raw(inode);
+	return inode_peek_iversion_raw(inode) >> I_VERSION_QUERIED_SHIFT;
 }
 
 /**
@@ -2213,12 +2241,28 @@ inode_peek_iversion(const struct inode *inode)
  * that a later query of the i_version will result in a different value if
  * anything has changed.
  *
- * This implementation just does a peek.
+ * In this implementation, we fetch the current value, set the QUERIED flag and
+ * then try to swap it into place with a cmpxchg, if it wasn't already set. If
+ * that fails, we try again with the newly fetched value from the cmpxchg.
  */
 static inline u64
 inode_query_iversion(struct inode *inode)
 {
-	return inode_peek_iversion(inode);
+	u64 cur, old, new;
+
+	cur = atomic64_read(&inode->i_version);
+	for (;;) {
+		/* If flag is already set, then no need to swap */
+		if (cur & I_VERSION_QUERIED)
+			break;
+
+		new = cur | I_VERSION_QUERIED;
+		old = atomic64_cmpxchg(&inode->i_version, cur, new);
+		if (old == cur)
+			break;
+		cur = old;
+	}
+	return cur >> I_VERSION_QUERIED_SHIFT;
 }
 
 /**
@@ -2228,7 +2272,11 @@ inode_query_iversion(struct inode *inode)
  *
  * Compare an i_version counter with a previous one. Returns 0 if they are
  * the same or non-zero if they are different.
+ *
+ * Note that we don't need to set the QUERIED flag in this case, as the value
+ * in the inode is not being recorded for later use.
  */
+
 static inline s64
 inode_cmp_iversion(const struct inode *inode, const u64 old)
 {
-- 
2.14.3