fs: change d_delete semantics

Change d_delete from a dentry deletion notification to a dentry caching
advise, more like ->drop_inode. Require it to be constant and idempotent,
and not take d_lock. This is how all existing filesystems use the callback
anyway.

This makes fine grained dentry locking of dput and dentry lru scanning
much simpler.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

1 files changed, 13 insertions, 14 deletions

diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index 20899e095e7e..95c0a93f056c 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -847,9 +847,9 @@ defined:
 
 struct dentry_operations {
         int (*d_revalidate)(struct dentry *, struct nameidata *);
-        int (*d_hash) (struct dentry *, struct qstr *);
-        int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
-        int (*d_delete)(struct dentry *);
+        int (*d_hash)(struct dentry *, struct qstr *);
+        int (*d_compare)(struct dentry *, struct qstr *, struct qstr *);
+        int (*d_delete)(const struct dentry *);
         void (*d_release)(struct dentry *);
         void (*d_iput)(struct dentry *, struct inode *);
         char *(*d_dname)(struct dentry *, char *, int);
@@ -864,9 +864,11 @@ struct dentry_operations {
 
   d_compare: called when a dentry should be compared with another
 
-  d_delete: called when the last reference to a dentry is
-        deleted. This means no-one is using the dentry, however it is
-        still valid and in the dcache
+  d_delete: called when the last reference to a dentry is dropped and the
+        dcache is deciding whether or not to cache it. Return 1 to delete
+        immediately, or 0 to cache the dentry. Default is NULL which means to
+        always cache a reachable dentry. d_delete must be constant and
+        idempotent.
 
   d_release: called when a dentry is really deallocated
 
@@ -910,14 +912,11 @@ manipulate dentries:
         the usage count)
 
   dput: close a handle for a dentry (decrements the usage count). If
-        the usage count drops to 0, the "d_delete" method is called
-        and the dentry is placed on the unused list if the dentry is
-        still in its parents hash list. Putting the dentry on the
-        unused list just means that if the system needs some RAM, it
-        goes through the unused list of dentries and deallocates them.
-        If the dentry has already been unhashed and the usage count
-        drops to 0, in this case the dentry is deallocated after the
-        "d_delete" method is called
+        the usage count drops to 0, and the dentry is still in its
+        parent's hash, the "d_delete" method is called to check whether
+        it should be cached. If it should not be cached, or if the dentry
+        is not hashed, it is deleted. Otherwise cached dentries are put
+        into an LRU list to be reclaimed on memory shortage.
 
   d_drop: this unhashes a dentry from its parents hash list. A
         subsequent call to dput() will deallocate the dentry if its