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.
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
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