[PATCH 1/3] VFS: Remove detached-dentry counter from shrink_dcache_for_umount_subtree()

[PATCH 1/3] VFS: Remove detached-dentry counter from shrink_dcache_for_umount_subtree()

[David Howells]

Remove the detached-dentry counter from shrink_dcache_for_umount_subtree() as
the value it computes is no longer used as of commit
312d3ca856d369bb04d0443846b85b4cdde6fa8a which made the nr_dentry counters
summed per-CPU rather than global atomic.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 fs/dcache.c |    3 ---
 1 files changed, 0 insertions(+), 3 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 37f72ee..9e02c6c 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -873,7 +873,6 @@ EXPORT_SYMBOL(shrink_dcache_sb);
 static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
 {
 	struct dentry *parent;
-	unsigned detached = 0;
 
 	BUG_ON(!IS_ROOT(dentry));
 
@@ -937,8 +936,6 @@ static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
 				spin_unlock(&parent->d_lock);
 			}
 
-			detached++;
-
 			inode = dentry->d_inode;
 			if (inode) {
 				dentry->d_inode = NULL;

[PATCH 2/3] VFS: Remove dentry->d_lock locking from shrink_dcache_for_umount_subtree()

[David Howells]

Locks of the dcache_lock were replaced by locks of dentry->d_lock in commits
such as:

	2304450783dfde7b0b94ae234edd0dbffa865073
	2fd6b7f50797f2e993eea59e0a0b8c6399c811dc

as part of the RCU-based pathwalk changes, despite the fact that the caller
(shrink_dcache_for_umount()) notes in the banner comment the reasons that
d_lock is not necessary in these functions:

/*
 * destroy the dentries attached to a superblock on unmounting
 * - we don't need to use dentry->d_lock because:
 *   - the superblock is detached from all mountings and open files, so the
 *     dentry trees will not be rearranged by the VFS
 *   - s_umount is write-locked, so the memory pressure shrinker will ignore
 *     any dentries belonging to this superblock that it comes across
 *   - the filesystem itself is no longer permitted to rearrange the dentries
 *     in this superblock
 */

So remove these locks.  If the locks are actually necessary, then this banner
comment should be altered instead.

The hash table chains are protected by 1-bit locks in the hash table heads, so
those shouldn't be a problem.

Note that to make this work, __d_drop() has to be split so that the RCUwalk
barrier can be avoided.  This causes problems otherwise as it has an assertion
that dentry->d_lock is locked - but there is no need for that as no one else
can be trying to access this dentry, except to step over it (and that should
be handled by d_free(), I think).

Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Nick Piggin <npiggin@kernel.dk>
---

 fs/dcache.c |   48 ++++++++++++++++++++++++------------------------
 1 files changed, 24 insertions(+), 24 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 9e02c6c..ed1e691 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -301,6 +301,27 @@ static struct dentry *d_kill(struct dentry *dentry, struct dentry *parent)
 	return parent;
 }
 
+/*
+ * Unhash a dentry without inserting an RCU walk barrier or checking that
+ * dentry->d_lock is locked.  The caller must take care of that, if
+ * appropriate.
+ */
+static void __d_shrink(struct dentry *dentry)
+{
+	if (!d_unhashed(dentry)) {
+		struct hlist_bl_head *b;
+		if (unlikely(dentry->d_flags & DCACHE_DISCONNECTED))
+			b = &dentry->d_sb->s_anon;
+		else
+			b = d_hash(dentry->d_parent, dentry->d_name.hash);
+
+		hlist_bl_lock(b);
+		__hlist_bl_del(&dentry->d_hash);
+		dentry->d_hash.pprev = NULL;
+		hlist_bl_unlock(b);
+	}
+}
+
 /**
  * d_drop - drop a dentry
  * @dentry: dentry to drop
@@ -319,17 +340,7 @@ static struct dentry *d_kill(struct dentry *dentry, struct dentry *parent)
 void __d_drop(struct dentry *dentry)
 {
 	if (!d_unhashed(dentry)) {
-		struct hlist_bl_head *b;
-		if (unlikely(dentry->d_flags & DCACHE_DISCONNECTED))
-			b = &dentry->d_sb->s_anon;
-		else
-			b = d_hash(dentry->d_parent, dentry->d_name.hash);
-
-		hlist_bl_lock(b);
-		__hlist_bl_del(&dentry->d_hash);
-		dentry->d_hash.pprev = NULL;
-		hlist_bl_unlock(b);
-
+		__d_shrink(dentry);
 		dentry_rcuwalk_barrier(dentry);
 	}
 }
@@ -877,10 +888,8 @@ static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
 	BUG_ON(!IS_ROOT(dentry));
 
 	/* detach this root from the system */
-	spin_lock(&dentry->d_lock);
 	dentry_lru_del(dentry);
-	__d_drop(dentry);
-	spin_unlock(&dentry->d_lock);
+	__d_shrink(dentry);
 
 	for (;;) {
 		/* descend to the first leaf in the current subtree */
@@ -889,16 +898,11 @@ static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
 
 			/* this is a branch with children - detach all of them
 			 * from the system in one go */
-			spin_lock(&dentry->d_lock);
 			list_for_each_entry(loop, &dentry->d_subdirs,
 					    d_u.d_child) {
-				spin_lock_nested(&loop->d_lock,
-						DENTRY_D_LOCK_NESTED);
 				dentry_lru_del(loop);
-				__d_drop(loop);
-				spin_unlock(&loop->d_lock);
+				__d_shrink(loop);
 			}
-			spin_unlock(&dentry->d_lock);
 
 			/* move to the first child */
 			dentry = list_entry(dentry->d_subdirs.next,
@@ -930,10 +934,8 @@ static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
 				list_del(&dentry->d_u.d_child);
 			} else {
 				parent = dentry->d_parent;
-				spin_lock(&parent->d_lock);
 				parent->d_count--;
 				list_del(&dentry->d_u.d_child);
-				spin_unlock(&parent->d_lock);
 			}
 
 			inode = dentry->d_inode;
@@ -980,9 +982,7 @@ void shrink_dcache_for_umount(struct super_block *sb)
 
 	dentry = sb->s_root;
 	sb->s_root = NULL;
-	spin_lock(&dentry->d_lock);
 	dentry->d_count--;
-	spin_unlock(&dentry->d_lock);
 	shrink_dcache_for_umount_subtree(dentry);
 
 	while (!hlist_bl_empty(&sb->s_anon)) {

[PATCH 3/3] VFS: Reorganise shrink_dcache_for_umount_subtree() after demise of dcache_lock

[David Howells]

Reorganise shrink_dcache_for_umount_subtree() in light of the demise of
dcache_lock.  Without that dcache_lock, there is no need for the batching of
removal of dentries from the system under it (we wanted to make intensive use
of the locked data whilst we held it, but didn't want to hold it for long at a
time).

This works, provided the preceding patch is correct in its removal of locking
on dentry->d_lock on the basis that no one should be locking these dentries any
more as the whole superblock is defunct.

With this patch, the calls to dentry_lru_del() and __d_shrink() are placed at
the point where each dentry is detached handled.

It is possible that, as an alternative, the batching should still be done -
but only for dentry_lru_del() of all a dentry's children in one go.  In such a
case, the batching would be done under dcache_lru_lock.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 fs/dcache.c |   22 +++++-----------------
 1 files changed, 5 insertions(+), 17 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index ed1e691..4b7b908 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -887,33 +887,21 @@ static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
 
 	BUG_ON(!IS_ROOT(dentry));
 
-	/* detach this root from the system */
-	dentry_lru_del(dentry);
-	__d_shrink(dentry);
-
 	for (;;) {
 		/* descend to the first leaf in the current subtree */
-		while (!list_empty(&dentry->d_subdirs)) {
-			struct dentry *loop;
-
-			/* this is a branch with children - detach all of them
-			 * from the system in one go */
-			list_for_each_entry(loop, &dentry->d_subdirs,
-					    d_u.d_child) {
-				dentry_lru_del(loop);
-				__d_shrink(loop);
-			}
-
-			/* move to the first child */
+		while (!list_empty(&dentry->d_subdirs))
 			dentry = list_entry(dentry->d_subdirs.next,
 					    struct dentry, d_u.d_child);
-		}
 
 		/* consume the dentries from this leaf up through its parents
 		 * until we find one with children or run out altogether */
 		do {
 			struct inode *inode;
 
+			/* detach from the system */
+			dentry_lru_del(dentry);
+			__d_shrink(dentry);
+
 			if (dentry->d_count != 0) {
 				printk(KERN_ERR
 				       "BUG: Dentry %p{i=%lx,n=%s}"