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

[Trond Myklebust <trondmy@hammerspace.com>]

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@suse.de>
> > > ---
> > >  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().

> 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@hammerspace.com

-- 
Trond Myklebust
CTO, Hammerspace Inc
4984 El Camino Real, Suite 208
Los Altos, CA 94022
​
www.hammer.space