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Date: Mon, 10 Oct 2011 09:19:43 -0700 (PDT)
From: Sage Weil <sage@...dream.net>
To: Dave Chinner <david@...morbit.com>
cc: Christoph Hellwig <hch@...radead.org>,
linux-fsdevel@...r.kernel.org, viro@...IV.linux.org.uk,
ceph-devel@...r.kernel.org, rwheeler@...hat.com,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfs: add d_prune dentry operation
On Mon, 10 Oct 2011, Dave Chinner wrote:
> On Thu, Oct 06, 2011 at 03:20:37PM -0700, Sage Weil wrote:
> > On Thu, 6 Oct 2011, Christoph Hellwig wrote:
> > > On Wed, Oct 05, 2011 at 09:26:10PM -0700, Sage Weil wrote:
> > > > This adds a d_prune dentry operation that is called by the VFS prior to
> > > > pruning (i.e. unhashing and killing) a hashed dentry from the dcache. This
> > > > will be used by Ceph to maintain a flag indicating whether the complete
> > > > contents of a directory are contained in the dcache, allowing it to satisfy
> > > > lookups and readdir without addition server communication.
> > >
> > > What tree is the patch against? I seems to fail to apply against Linus'
> > > latests.
> >
> > Whoops, I rebased against v3.0 instead of latest master.
> >
> > > It also seem like it basically should not be be opencoded but in a
> > > wrapper around dentry_lru_del for all cases but the lazy removal from
> > > LRU in case that is referenced, with some comments explaining the whole
> > > thing.
> >
> > Yeah, that's a bit better. There are four dentry_lru_del() callers, two
> > where we there is a reference, and two where we want to ->d_prune too.
>
> The code in the patch doesn't explain to me why you'd need to call
> dentry_lru_prune() rather than dentry_lru_del()? It's something to
> do with the difference between active and inactive LRU removal, but
> I can't really tell. My patchset removes the LRU abuse from
> select_parent, so I'm kind of wondering what the correct thing is to
> do there.
>
> Hence, can you add a bit of documentation to those functions
> explaining why and when you should use one or the other? i.e
> document the situations where the FS needs to be notified of
> pruning, rather than leaving anyone who is reading the code
> guessing.
Let me know if the below is clearer. The requirement is that ->d_prune be
called prior to unhashing (and then destroying) a victim dentry. I'm a
little worried about mixing that in with the lru helpers because it is
only indirectly related to whether the dentry is on the LRU, and that may
confuse people. A cleaned up opencoded patch is here:
https://github.com/NewDreamNetwork/ceph-client/commit/784a6aa6dc7baf2069c40988d79130dba17c7068
and the updated dentry_lru_prune() wrapper version is below.
Thanks-
sage
>From 619707e0b6fca69f165d1d4f048ab8211531d559 Mon Sep 17 00:00:00 2001
From: Sage Weil <sage@...dream.net>
Date: Mon, 10 Oct 2011 09:18:16 -0700
Subject: [PATCH] vfs: add d_prune dentry operation
This adds a d_prune dentry operation that is called by the VFS prior to
pruning (i.e. unhashing and killing) a hashed dentry from the dcache.
Wrap dentry_lru_del() and use the new _prune() helper in the cases where we
are about to unhash and kill the dentry.
This will be used by Ceph to maintain a flag indicating whether the
complete contents of a directory are contained in the dcache, allowing it
to satisfy lookups and readdir without addition server communication.
Signed-off-by: Sage Weil <sage@...dream.net>
---
Documentation/filesystems/Locking | 1 +
fs/dcache.c | 40 ++++++++++++++++++++++++++++++++----
include/linux/dcache.h | 3 ++
3 files changed, 39 insertions(+), 5 deletions(-)
diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index 6533807..d819ba1 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -29,6 +29,7 @@ d_hash no no no maybe
d_compare: yes no no maybe
d_delete: no yes no no
d_release: no no yes no
+d_prune: no yes no no
d_iput: no no yes no
d_dname: no no no no
d_automount: no no yes no
diff --git a/fs/dcache.c b/fs/dcache.c
index a88948b..274f13e 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -225,7 +225,7 @@ static void dentry_unlink_inode(struct dentry * dentry)
}
/*
- * dentry_lru_(add|del|move_tail) must be called with d_lock held.
+ * dentry_lru_(add|del|prune|move_tail) must be called with d_lock held.
*/
static void dentry_lru_add(struct dentry *dentry)
{
@@ -245,6 +245,9 @@ static void __dentry_lru_del(struct dentry *dentry)
dentry_stat.nr_unused--;
}
+/*
+ * Remove a dentry with references from the LRU.
+ */
static void dentry_lru_del(struct dentry *dentry)
{
if (!list_empty(&dentry->d_lru)) {
@@ -254,6 +257,23 @@ static void dentry_lru_del(struct dentry *dentry)
}
}
+/*
+ * Remove a dentry that is unreferenced and about to be pruned
+ * (unhashed and destroyed) from the LRU, and inform the file system.
+ * This wrapper should be called _prior_ to unhashing a victim dentry.
+ */
+static void dentry_lru_prune(struct dentry *dentry)
+{
+ if (!list_empty(&dentry->d_lru)) {
+ if (dentry->d_flags & DCACHE_OP_PRUNE)
+ dentry->d_op->d_prune(dentry);
+
+ spin_lock(&dcache_lru_lock);
+ __dentry_lru_del(dentry);
+ spin_unlock(&dcache_lru_lock);
+ }
+}
+
static void dentry_lru_move_tail(struct dentry *dentry)
{
spin_lock(&dcache_lru_lock);
@@ -403,8 +423,12 @@ relock:
if (ref)
dentry->d_count--;
- /* if dentry was on the d_lru list delete it from there */
- dentry_lru_del(dentry);
+ /*
+ * if dentry was on the d_lru list delete it from there.
+ * inform the fs via d_prune that this dentry is about to be
+ * unhashed and destroyed.
+ */
+ dentry_lru_prune(dentry);
/* if it was on the hash then remove it */
__d_drop(dentry);
return d_kill(dentry, parent);
@@ -854,8 +878,12 @@ static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
do {
struct inode *inode;
- /* detach from the system */
- dentry_lru_del(dentry);
+ /*
+ * remove the dentry from the lru, and inform
+ * the fs that this dentry is about to be
+ * unhashed and destroyed.
+ */
+ dentry_lru_prune(dentry);
__d_shrink(dentry);
if (dentry->d_count != 0) {
@@ -1283,6 +1311,8 @@ void d_set_d_op(struct dentry *dentry, const struct dentry_operations *op)
dentry->d_flags |= DCACHE_OP_REVALIDATE;
if (op->d_delete)
dentry->d_flags |= DCACHE_OP_DELETE;
+ if (op->d_prune)
+ dentry->d_flags |= DCACHE_OP_PRUNE;
}
EXPORT_SYMBOL(d_set_d_op);
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 62157c0..69ee43a 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -165,6 +165,7 @@ struct dentry_operations {
unsigned int, const char *, const struct qstr *);
int (*d_delete)(const struct dentry *);
void (*d_release)(struct dentry *);
+ void (*d_prune)(struct dentry *);
void (*d_iput)(struct dentry *, struct inode *);
char *(*d_dname)(struct dentry *, char *, int);
struct vfsmount *(*d_automount)(struct path *);
@@ -216,6 +217,8 @@ struct dentry_operations {
#define DCACHE_MANAGED_DENTRY \
(DCACHE_MOUNTED|DCACHE_NEED_AUTOMOUNT|DCACHE_MANAGE_TRANSIT)
+#define DCACHE_OP_PRUNE 0x80000
+
extern seqlock_t rename_lock;
static inline int dname_external(struct dentry *dentry)
--
1.7.0
--
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