[PATCH 1/2] dcache: use IS_ROOT to decide where dentry is hashed

[PATCH 1/2] dcache: use IS_ROOT to decide where dentry is hashed

[J. Bruce Fields]

From: "J. Bruce Fields" <bfields@redhat.com>

Every hashed dentry is either hashed in the dentry_hashtable, or a
superblock's s_anon list.

__d_shrink assumes it can determine which is the case by checking
DCACHE_DISCONNECTED; this is not true.

It is true that when DCACHE_DISCONNECTED is cleared, the dentry is not
only hashed on dentry_hashtable, but is fully connected to its parents
back to the root.

But the converse is *not* true: fs/exportfs/expfs.c:reconnect_path()
attempts to connect a directory (found by filehandle lookup) back to
root by ascending to parents and performing lookups one at a time.  It
does not clear DCACHE_DISCONNECTED until its done, and that is not at
all an atomic process.

In particular, it is possible for DCACHE_DISCONNECTED to be set on a
dentry which is hashed on the dentry_hashtable.

Instead, use IS_ROOT() to check which hash chain a dentry is on.  This
*does* work:

Dentries are hashed only by:

	- d_obtain_alias, which adds an IS_ROOT() dentry to sb_anon.

	- __d_rehash, called by _d_rehash: hashes to the dentry's
	  parent, and all callers of _d_rehash appear to have d_parent
	  set to a "real" parent.
	- __d_rehash, called by __d_move: rehashes the moved dentry to
	  hash chain determined by target, and assigns target's d_parent
	  to its d_parent, before dropping the dentry's d_lock.

Therefore I believe it's safe for a holder of a dentry's d_lock to
assume that it is hashed on sb_anon if and only if IS_ROOT(dentry) is
true.

I believe the incorrect assumption about DCACHE_DISCONNECTED was
originally introduced by ceb5bdc2d246 "fs: dcache per-bucket dcache hash
locking".

Cc: Nick Piggin <npiggin@kernel.dk>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
---
 fs/dcache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 4046904..740b4f5 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -410,7 +410,7 @@ static void __d_shrink(struct dentry *dentry)
 {
 	if (!d_unhashed(dentry)) {
 		struct hlist_bl_head *b;
-		if (unlikely(dentry->d_flags & DCACHE_DISCONNECTED))
+		if (unlikely(IS_ROOT(dentry)))
 			b = &dentry->d_sb->s_anon;
 		else
 			b = d_hash(dentry->d_parent, dentry->d_name.hash);
-- 
1.7.11.2

[PATCH 1/2] dcache: use IS_ROOT to decide where dentry is hashed

[J. Bruce Fields]

From: "J. Bruce Fields" <bfields-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>

Every hashed dentry is either hashed in the dentry_hashtable, or a
superblock's s_anon list.

__d_shrink assumes it can determine which is the case by checking
DCACHE_DISCONNECTED; this is not true.

It is true that when DCACHE_DISCONNECTED is cleared, the dentry is not
only hashed on dentry_hashtable, but is fully connected to its parents
back to the root.

But the converse is *not* true: fs/exportfs/expfs.c:reconnect_path()
attempts to connect a directory (found by filehandle lookup) back to
root by ascending to parents and performing lookups one at a time.  It
does not clear DCACHE_DISCONNECTED until its done, and that is not at
all an atomic process.

In particular, it is possible for DCACHE_DISCONNECTED to be set on a
dentry which is hashed on the dentry_hashtable.

Instead, use IS_ROOT() to check which hash chain a dentry is on.  This
*does* work:

Dentries are hashed only by:

	- d_obtain_alias, which adds an IS_ROOT() dentry to sb_anon.

	- __d_rehash, called by _d_rehash: hashes to the dentry's
	  parent, and all callers of _d_rehash appear to have d_parent
	  set to a "real" parent.
	- __d_rehash, called by __d_move: rehashes the moved dentry to
	  hash chain determined by target, and assigns target's d_parent
	  to its d_parent, before dropping the dentry's d_lock.

Therefore I believe it's safe for a holder of a dentry's d_lock to
assume that it is hashed on sb_anon if and only if IS_ROOT(dentry) is
true.

I believe the incorrect assumption about DCACHE_DISCONNECTED was
originally introduced by ceb5bdc2d246 "fs: dcache per-bucket dcache hash
locking".

Cc: Nick Piggin <npiggin-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org>
Reviewed-by: NeilBrown <neilb-l3A5Bk7waGM@public.gmane.org>
Signed-off-by: J. Bruce Fields <bfields-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
---
 fs/dcache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 4046904..740b4f5 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -410,7 +410,7 @@ static void __d_shrink(struct dentry *dentry)
 {
 	if (!d_unhashed(dentry)) {
 		struct hlist_bl_head *b;
-		if (unlikely(dentry->d_flags & DCACHE_DISCONNECTED))
+		if (unlikely(IS_ROOT(dentry)))
 			b = &dentry->d_sb->s_anon;
 		else
 			b = d_hash(dentry->d_parent, dentry->d_name.hash);
-- 
1.7.11.2

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[PATCH 2/2] nfsd4: Don't set DISCONNECTED on "pseudo filesystem" dentries

[J. Bruce Fields]

From: "J. Bruce Fields" <bfields@redhat.com>

I can't for the life of me see any reason why anyone would care whether
a dentry that is never hooked into the dentry cache would need
DCACHE_DISCONNECTED set.

This originates from 4b936885ab04dc6e0bb0ef35e0e23c1a7364d9e5 "fs:
improve scalability of pseudo filesystems", which probably just made the
false assumption the DCACHE_DISCONNECTED was meant to be set on anything
not connected to a parent somehow.

So this is just confusing.  Ideally the only use DCACHE_DISCONNECTED
would be in the filehandle-lookup code, which needs it to ensure
dentries are connected into the dentry tree before use.

I left d_alloc_pseudo there even though it's now equivalent to
__d_alloc(), just on the theory the name is better documentation of its
intended use outside dcache.c.

Cc: Nick Piggin <npiggin@kernel.dk>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
---
 fs/dcache.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 740b4f5..1b1e925 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1350,12 +1350,13 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
 }
 EXPORT_SYMBOL(d_alloc);
 
+/*
+ * For filesystems that do not actually use the dentry cache at all, and
+ * only ever deal in IS_ROOT() dentries:
+ */
 struct dentry *d_alloc_pseudo(struct super_block *sb, const struct qstr *name)
 {
-	struct dentry *dentry = __d_alloc(sb, name);
-	if (dentry)
-		dentry->d_flags |= DCACHE_DISCONNECTED;
-	return dentry;
+	return __d_alloc(sb, name);
 }
 EXPORT_SYMBOL(d_alloc_pseudo);
 
-- 
1.7.11.2