| Message ID | 162306072498.69474.16160057168984328507.stgit@web.messagingengine.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | kernfs: proposed locking and concurrency improvement | expand |
Ian Kent <raven@themaw.net> writes: > If there are many lookups for non-existent paths these negative lookups > can lead to a lot of overhead during path walks. > > The VFS allows dentries to be created as negative and hashed, and caches > them so they can be used to reduce the fairly high overhead alloc/free > cycle that occurs during these lookups. > > Use the kernfs node parent revision to identify if a change has been > made to the containing directory so that the negative dentry can be > discarded and the lookup redone. > > Signed-off-by: Ian Kent <raven@themaw.net> > --- > fs/kernfs/dir.c | 53 +++++++++++++++++++++++++++++++---------------------- > 1 file changed, 31 insertions(+), 22 deletions(-) > > diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c > index b88432c48851f..5ae95e8d1aea1 100644 > --- a/fs/kernfs/dir.c > +++ b/fs/kernfs/dir.c > @@ -1039,13 +1039,32 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags) > if (flags & LOOKUP_RCU) > return -ECHILD; > > - /* Always perform fresh lookup for negatives */ > - if (d_really_is_negative(dentry)) > - goto out_bad_unlocked; > - > kn = kernfs_dentry_node(dentry); > mutex_lock(&kernfs_mutex); > > + /* Negative hashed dentry? */ > + if (!kn) { > + struct dentry *d_parent = dget_parent(dentry); > + struct kernfs_node *parent; > + > + /* If the kernfs parent node has changed discard and > + * proceed to ->lookup. > + */ > + parent = kernfs_dentry_node(d_parent); > + if (parent) { > + if (kernfs_dir_changed(parent, dentry)) { > + dput(d_parent); > + goto out_bad; > + } > + } > + dput(d_parent); > + > + /* The kernfs node doesn't exist, leave the dentry > + * negative and return success. > + */ > + goto out; > + } What part of this new negative hashed dentry check needs the kernfs_mutex? I guess it is the reading of kn->dir.rev. Since all you are doing is comparing if two fields are equal it really should not matter. Maybe somewhere there needs to be a sprinkling of primitives like READ_ONCE. It just seems like such a waste to put all of that under kernfs_mutex on the off chance kn->dir.rev will change while it is being read. Eric
On Mon, 2021-06-07 at 13:27 -0500, Eric W. Biederman wrote: > Ian Kent <raven@themaw.net> writes: > > > If there are many lookups for non-existent paths these negative > > lookups > > can lead to a lot of overhead during path walks. > > > > The VFS allows dentries to be created as negative and hashed, and > > caches > > them so they can be used to reduce the fairly high overhead > > alloc/free > > cycle that occurs during these lookups. > > > > Use the kernfs node parent revision to identify if a change has > > been > > made to the containing directory so that the negative dentry can be > > discarded and the lookup redone. > > > > Signed-off-by: Ian Kent <raven@themaw.net> > > --- > > fs/kernfs/dir.c | 53 +++++++++++++++++++++++++++++++------------ > > ---------- > > 1 file changed, 31 insertions(+), 22 deletions(-) > > > > diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c > > index b88432c48851f..5ae95e8d1aea1 100644 > > --- a/fs/kernfs/dir.c > > +++ b/fs/kernfs/dir.c > > @@ -1039,13 +1039,32 @@ static int kernfs_dop_revalidate(struct > > dentry *dentry, unsigned int flags) > > if (flags & LOOKUP_RCU) > > return -ECHILD; > > > > - /* Always perform fresh lookup for negatives */ > > - if (d_really_is_negative(dentry)) > > - goto out_bad_unlocked; > > - > > kn = kernfs_dentry_node(dentry); > > mutex_lock(&kernfs_mutex); > > > > + /* Negative hashed dentry? */ > > + if (!kn) { > > + struct dentry *d_parent = dget_parent(dentry); > > + struct kernfs_node *parent; > > + > > + /* If the kernfs parent node has changed discard > > and > > + * proceed to ->lookup. > > + */ > > + parent = kernfs_dentry_node(d_parent); > > + if (parent) { > > + if (kernfs_dir_changed(parent, dentry)) { > > + dput(d_parent); > > + goto out_bad; > > + } > > + } > > + dput(d_parent); > > + > > + /* The kernfs node doesn't exist, leave the dentry > > + * negative and return success. > > + */ > > + goto out; > > + } > > What part of this new negative hashed dentry check needs the > kernfs_mutex? > > I guess it is the reading of kn->dir.rev. I have an irresistible urge to keep the rb tree stable when accessing it. It was probably not necessary most of the times I did it, IIUC even a rebalance will leave the node address unchanged so it should be just removals and moves to worry about. > > Since all you are doing is comparing if two fields are equal it > really should not matter. Maybe somewhere there needs to be a > sprinkling of primitives like READ_ONCE. There is one case that looks tricky, rename will call ->rename() and a bit later do the move. Thinking about it a READ_ONCE might be needed even now but taking the rwsem is probably enough. Not sure about that one? Moving this out from under the rwsem would be good to do. Ian > > It just seems like such a waste to put all of that under kernfs_mutex > on the off chance kn->dir.rev will change while it is being read. > > Eric
diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c index b88432c48851f..5ae95e8d1aea1 100644 --- a/fs/kernfs/dir.c +++ b/fs/kernfs/dir.c @@ -1039,13 +1039,32 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags) if (flags & LOOKUP_RCU) return -ECHILD; - /* Always perform fresh lookup for negatives */ - if (d_really_is_negative(dentry)) - goto out_bad_unlocked; - kn = kernfs_dentry_node(dentry); mutex_lock(&kernfs_mutex); + /* Negative hashed dentry? */ + if (!kn) { + struct dentry *d_parent = dget_parent(dentry); + struct kernfs_node *parent; + + /* If the kernfs parent node has changed discard and + * proceed to ->lookup. + */ + parent = kernfs_dentry_node(d_parent); + if (parent) { + if (kernfs_dir_changed(parent, dentry)) { + dput(d_parent); + goto out_bad; + } + } + dput(d_parent); + + /* The kernfs node doesn't exist, leave the dentry + * negative and return success. + */ + goto out; + } + /* The kernfs node has been deactivated */ if (!kernfs_active(kn)) goto out_bad; @@ -1062,12 +1081,11 @@ 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: mutex_unlock(&kernfs_mutex); return 1; out_bad: mutex_unlock(&kernfs_mutex); -out_bad_unlocked: return 0; } @@ -1082,30 +1100,21 @@ 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; mutex_lock(&kernfs_mutex); - if (kernfs_ns_enabled(parent)) ns = kernfs_info(dir->i_sb)->ns; kn = kernfs_find_ns(parent, dentry->d_name.name, ns); - - /* no such entry */ - if (!kn || !kernfs_active(kn)) { - ret = NULL; - goto out_unlock; - } - /* attach dentry and inode */ - inode = kernfs_get_inode(dir->i_sb, kn); - if (!inode) { - ret = ERR_PTR(-ENOMEM); - goto out_unlock; + if (kn && kernfs_active(kn)) { + inode = kernfs_get_inode(dir->i_sb, kn); + if (!inode) + inode = ERR_PTR(-ENOMEM); } - - /* instantiate and hash dentry */ + /* instantiate and hash (possibly negative) dentry */ ret = d_splice_alias(inode, dentry); if (!IS_ERR(ret)) { if (unlikely(ret)) @@ -1113,8 +1122,8 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir, else kernfs_set_rev(parent, dentry); } - out_unlock: mutex_unlock(&kernfs_mutex); + return ret; }
If there are many lookups for non-existent paths these negative lookups can lead to a lot of overhead during path walks. The VFS allows dentries to be created as negative and hashed, and caches them so they can be used to reduce the fairly high overhead alloc/free cycle that occurs during these lookups. Use the kernfs node parent revision to identify if a change has been made to the containing directory so that the negative dentry can be discarded and the lookup redone. Signed-off-by: Ian Kent <raven@themaw.net> --- fs/kernfs/dir.c | 53 +++++++++++++++++++++++++++++++---------------------- 1 file changed, 31 insertions(+), 22 deletions(-)