Re: [PATCH] NFS: only invalidate dentrys that are clearly invalid.

[Trond Myklebust]

On Mon, 2020-11-16 at 16:00 +1100, NeilBrown wrote:
> On Mon, Nov 16 2020, Trond Myklebust wrote:
> 
> > On Mon, 2020-11-16 at 15:43 +1100, NeilBrown wrote:
> > > On Mon, Nov 16 2020, Trond Myklebust wrote:
> > > 
> > > > On Mon, 2020-11-16 at 13:59 +1100, NeilBrown wrote:
> > > > > 
> > > > > Prior to commit 5ceb9d7fdaaf ("NFS: Refactor
> > > > > nfs_lookup_revalidate()")
> > > > > and error from nfs_lookup_verify_inode() other than -ESTALE
> > > > > would
> > > > > result
> > > > > in nfs_lookup_revalidate() returning that error code (-ESTALE
> > > > > is
> > > > > mapped
> > > > > to zero).
> > > > > Since that commit, all errors result in zero being returned.
> > > > > 
> > > > > When nfs_lookup_revalidate() returns zero, the dentry is
> > > > > invalidated
> > > > > and, significantly, if the dentry is a directory that is
> > > > > mounted
> > > > > on,
> > > > > that mountpoint is lost.
> > > > > 
> > > > > If you:
> > > > >  - mount an NFS filesystem which contains a directory
> > > > >  - mount something (e.g. tmpfs) on that directory
> > > > >  - use iptables (or scissors) to block traffic to the server
> > > > >  - ls -l the-mounted-on-directory
> > > > >  - interrupt the 'ls -l'
> > > > > you will find that the directory has been unmounted.
> > > > > 
> > > > > This can be fixed by returning the actual error code from
> > > > > nfs_lookup_verify_inode() rather then zero (except for -
> > > > > ESTALE).
> > > > > 
> > > > > Fixes: 5ceb9d7fdaaf ("NFS: Refactor nfs_lookup_revalidate()")
> > > > > Signed-off-by: NeilBrown <neilb@xxxxxxx>
> > > > > ---
> > > > >  fs/nfs/dir.c | 8 +++++---
> > > > >  1 file changed, 5 insertions(+), 3 deletions(-)
> > > > > 
> > > > > diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
> > > > > index cb52db9a0cfb..d24acf556e9e 100644
> > > > > --- a/fs/nfs/dir.c
> > > > > +++ b/fs/nfs/dir.c
> > > > > @@ -1350,7 +1350,7 @@ nfs_do_lookup_revalidate(struct inode
> > > > > *dir,
> > > > > struct dentry *dentry,
> > > > >                          unsigned int flags)
> > > > >  {
> > > > >         struct inode *inode;
> > > > > -       int error;
> > > > > +       int error = 0;
> > > > >  
> > > > >         nfs_inc_stats(dir, NFSIOS_DENTRYREVALIDATE);
> > > > >         inode = d_inode(dentry);
> > > > > @@ -1372,8 +1372,10 @@ nfs_do_lookup_revalidate(struct inode
> > > > > *dir,
> > > > > struct dentry *dentry,
> > > > >             nfs_check_verifier(dir, dentry, flags &
> > > > > LOOKUP_RCU))
> > > > > {
> > > > >                 error = nfs_lookup_verify_inode(inode,
> > > > > flags);
> > > > >                 if (error) {
> > > > > -                       if (error == -ESTALE)
> > > > > +                       if (error == -ESTALE) {
> > > > >                                 nfs_zap_caches(dir);
> > > > > +                               error = 0;
> > > > > +                       }
> > > > >                         goto out_bad;
> > > > >                 }
> > > > >                 nfs_advise_use_readdirplus(dir);
> > > > > @@ -1395,7 +1397,7 @@ nfs_do_lookup_revalidate(struct inode
> > > > > *dir,
> > > > > struct dentry *dentry,
> > > > >  out_bad:
> > > > >         if (flags & LOOKUP_RCU)
> > > > >                 return -ECHILD;
> > > > > -       return nfs_lookup_revalidate_done(dir, dentry, inode,
> > > > > 0);
> > > > > +       return nfs_lookup_revalidate_done(dir, dentry, inode,
> > > > > error);
> > > > 
> > > > Which errors do we actually need to return here? As far as I
> > > > can
> > > > tell,
> > > > the only errors that nfs_lookup_verify_inode() is supposed to
> > > > return is
> > > > ENOMEM, ESTALE, ECHILD, and possibly EIO or ETiMEDOUT.
> > > > 
> > > > Why would it be better to return those errors rather than just
> > > > a 0
> > > > when
> > > > we need to invalidate the inode, particularly since we already
> > > > have
> > > > a
> > > > special case in nfs_lookup_revalidate_done() when the dentry is
> > > > root?
> > > 
> > > ERESTARTSYS is the error that easily causes problems.
> > > 
> > > Returning 0 causes d_invalidate() to be called which is quite
> > > heavy
> > > handed in mountpoints.
> > 
> > My point is that it shouldn't get returned for mountpoints. See
> > nfs_lookup_revalidate_done().
> 
> nfs_lookup_revalidate_done() only checks IS_ROOT(), and while many
> mountpoints are IS_ROOT(), not all are (--bind easily makes others).
> 
> But that isn't even really relevant here.  The dentry being
> revalidated
> is the underlying directory - that something else is mounted on.
> step_into() which follows mount points is called in walk_component()
> *after* lookup_fast or lookup_slow which will have revalidated the
> dentry.

So then why is it not sufficient to just add a check for
d_mountpoint()? This is a revalidation, not a new lookup.

> 
> NeilBrown
> 
> 
> > 
> > > So it is only reasonable to return 0 when we have unambiguous
> > > confirmation from the server that the object no longer exists. 
> > > ESTALE
> > > is unambiguous. EIO might be unambiguous.  ERESTARTSYS, ENOMEM,
> > > ETIMEDOUT are transient and don't justify d_invalidate() being
> > > called.
> > > 
> > > (BTW, Commit cc89684c9a26 ("NFS: only invalidate dentrys that are
> > > clearly invalid.")
> > >  fixed much the same bug 3 years ago).
> > >  
> > > Thanks,
> > > NeilBrown
> > > 
> > > 
> > > > 
> > > > >  }
> > > > >  
> > > > >  static int
> > > > 
> > > > -- 
> > > > Trond Myklebust
> > > > Linux NFS client maintainer, Hammerspace
> > > > trond.myklebust@xxxxxxxxxxxxxxx
> > 
> > -- 
> > Trond Myklebust
> > CTO, Hammerspace Inc
> > 4984 El Camino Real, Suite 208
> > Los Altos, CA 94022
> > ​
> > www.hammer.space

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@xxxxxxxxxxxxxxx