From: Ian Kent <raven@themaw.net>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Tejun Heo <tj@kernel.org>,
Rick Lindsley <ricklind@linux.vnet.ibm.com>,
Stephen Rothwell <sfr@canb.auug.org.au>,
David Howells <dhowells@redhat.com>,
Miklos Szeredi <miklos@szeredi.hu>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: [PATCH 3/4] kernfs: improve kernfs path resolution
Date: Mon, 25 May 2020 13:47:14 +0800 [thread overview]
Message-ID: <159038563473.276051.9549849659872866062.stgit@mickey.themaw.net> (raw)
In-Reply-To: <159038508228.276051.14042452586133971255.stgit@mickey.themaw.net>
Now that an rwsem is used by kernfs, take advantage of it to reduce
lookup overhead.
If there are many lookups (possibly many negative ones) there can
be a lot of overhead during path walks.
To reduce lookup overhead avoid allocating a new dentry where possible.
To do this stay in rcu-walk mode where possible and use the dentry cache
handling of negative hashed dentries to avoid allocating (and freeing
shortly after) new dentries on every negative lookup.
Signed-off-by: Ian Kent <raven@themaw.net>
---
fs/kernfs/dir.c | 87 ++++++++++++++++++++++++++++++++++++++++++++++---------
1 file changed, 72 insertions(+), 15 deletions(-)
diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 9b315f3b20ee..f4943329e578 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -1046,15 +1046,75 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
{
struct kernfs_node *kn;
- if (flags & LOOKUP_RCU)
+ if (flags & LOOKUP_RCU) {
+ kn = kernfs_dentry_node(dentry);
+ if (!kn) {
+ /* Negative hashed dentry, tell the VFS to switch to
+ * ref-walk mode and call us again so that node
+ * existence can be checked.
+ */
+ if (!d_unhashed(dentry))
+ return -ECHILD;
+
+ /* Negative unhashed dentry, this shouldn't happen
+ * because this case occurs in rcu-walk mode after
+ * dentry allocation which is followed by a call
+ * to ->loopup(). But if it does happen the dentry
+ * is surely invalid.
+ */
+ return 0;
+ }
+
+ /* Since the dentry is positive (we got the kernfs node) a
+ * kernfs node reference was held at the time. Now if the
+ * dentry reference count is still greater than 0 it's still
+ * positive so take a reference to the node to perform an
+ * active check.
+ */
+ if (d_count(dentry) <= 0 || !atomic_inc_not_zero(&kn->count))
+ return -ECHILD;
+
+ /* The kernfs node reference count was greater than 0, if
+ * it's active continue in rcu-walk mode.
+ */
+ if (kernfs_active_read(kn)) {
+ kernfs_put(kn);
+ return 1;
+ }
+
+ /* Otherwise, just tell the VFS to switch to ref-walk mode
+ * and call us again so the kernfs node can be validated.
+ */
+ kernfs_put(kn);
return -ECHILD;
+ }
- /* Always perform fresh lookup for negatives */
- if (d_really_is_negative(dentry))
- goto out_bad_unlocked;
+ down_read(&kernfs_rwsem);
kn = kernfs_dentry_node(dentry);
- down_read(&kernfs_rwsem);
+ if (!kn) {
+ struct kernfs_node *parent;
+
+ /* If the kernfs node can be found this is a stale negative
+ * hashed dentry so it must be discarded and the lookup redone.
+ */
+ parent = kernfs_dentry_node(dentry->d_parent);
+ if (parent) {
+ const void *ns = NULL;
+
+ if (kernfs_ns_enabled(parent))
+ ns = kernfs_info(dentry->d_parent->d_sb)->ns;
+ kn = kernfs_find_ns(parent, dentry->d_name.name, ns);
+ if (kn)
+ goto out_bad;
+ }
+
+ /* The kernfs node doesn't exist, leave the dentry negative
+ * and return success.
+ */
+ goto out;
+ }
+
/* The kernfs node has been deactivated */
if (!kernfs_active_read(kn))
@@ -1072,12 +1132,11 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
if (kn->parent && kernfs_ns_enabled(kn->parent) &&
kernfs_info(dentry->d_sb)->ns != kn->ns)
goto out_bad;
-
+out:
up_read(&kernfs_rwsem);
return 1;
out_bad:
up_read(&kernfs_rwsem);
-out_bad_unlocked:
return 0;
}
@@ -1092,7 +1151,7 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,
struct dentry *ret;
struct kernfs_node *parent = dir->i_private;
struct kernfs_node *kn;
- struct inode *inode;
+ struct inode *inode = NULL;
const void *ns = NULL;
down_read(&kernfs_rwsem);
@@ -1102,11 +1161,9 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,
kn = kernfs_find_ns(parent, dentry->d_name.name, ns);
- /* no such entry */
- if (!kn || !kernfs_active(kn)) {
- ret = NULL;
- goto out_unlock;
- }
+ /* no such entry, retain as negative hashed dentry */
+ if (!kn || !kernfs_active(kn))
+ goto out_negative;
/* attach dentry and inode */
inode = kernfs_get_inode(dir->i_sb, kn);
@@ -1114,10 +1171,10 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,
ret = ERR_PTR(-ENOMEM);
goto out_unlock;
}
-
+out_negative:
/* instantiate and hash dentry */
ret = d_splice_alias(inode, dentry);
- out_unlock:
+out_unlock:
up_read(&kernfs_rwsem);
return ret;
}
next prev parent reply other threads:[~2020-05-25 5:47 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-25 5:46 [PATCH 0/4] kernfs: proposed locking and concurrency improvement Ian Kent
2020-05-25 5:47 ` [PATCH 1/4] kernfs: switch kernfs to use an rwsem Ian Kent
2020-06-06 15:52 ` [kernfs] ea7c5fc39a: stress-ng.stream.ops_per_sec 11827.2% improvement kernel test robot
2020-06-06 18:18 ` Greg Kroah-Hartman
2020-06-07 1:13 ` Ian Kent
2020-06-11 2:06 ` kernel test robot
2020-06-11 2:20 ` Rick Lindsley
2020-06-11 3:02 ` Ian Kent
2020-06-07 8:40 ` [PATCH 1/4] kernfs: switch kernfs to use an rwsem Ian Kent
2020-06-08 9:58 ` Ian Kent
2020-05-25 5:47 ` [PATCH 2/4] kernfs: move revalidate to be near lookup Ian Kent
2020-05-25 5:47 ` Ian Kent [this message]
2020-05-25 5:47 ` [PATCH 4/4] kernfs: use revision to identify directory node changes Ian Kent
2020-05-25 6:16 ` [PATCH 0/4] kernfs: proposed locking and concurrency improvement Greg Kroah-Hartman
2020-05-25 7:23 ` Ian Kent
2020-05-25 7:31 ` Greg Kroah-Hartman
2020-05-27 12:44 ` Rick Lindsley
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