On Thu, 3 Jun 2021 at 19:26, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote: > > Ian Kent <raven@xxxxxxxxxx> 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. > > > > Signed-off-by: Ian Kent <raven@xxxxxxxxxx> > > --- > > fs/kernfs/dir.c | 55 +++++++++++++++++++++++++++++++++---------------------- > > 1 file changed, 33 insertions(+), 22 deletions(-) > > > > diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c > > index 4c69e2af82dac..5151c712f06f5 100644 > > --- a/fs/kernfs/dir.c > > +++ b/fs/kernfs/dir.c > > @@ -1037,12 +1037,33 @@ 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; > > + mutex_lock(&kernfs_mutex); > > > > kn = kernfs_dentry_node(dentry); > > - mutex_lock(&kernfs_mutex); > > Why bring kernfs_dentry_node inside the mutex? > > The inode lock of the parent should protect negative to positive > transitions not the kernfs_mutex. So moving the code inside > the mutex looks unnecessary and confusing. Except that d_revalidate() may or may not be called with parent lock held. Thanks, Miklos
