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From:   NeilBrown <neilb@suse.de>
To:     Trond Myklebust <trondmy@hammerspace.com>,
        "anna.schumaker@netapp.com" <anna.schumaker@netapp.com>
Date:   Mon, 16 Nov 2020 16:12:32 +1100
Cc:     "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] NFS: only invalidate dentrys that are clearly invalid.
In-Reply-To: <b28216153179dd20c22aa164259d3f901099896c.camel@hammerspace.com>
References: <87361aovm3.fsf@notabene.neil.brown.name>
 <d2fabd4b78dda3bd52519b84f50785dbcc2d40fb.camel@hammerspace.com>
 <87zh3hoqrx.fsf@notabene.neil.brown.name>
 <d208c9c085d8abf27a764e31a61e98f9c3743675.camel@hammerspace.com>
 <87wnylopyv.fsf@notabene.neil.brown.name>
 <b28216153179dd20c22aa164259d3f901099896c.camel@hammerspace.com>
Message-ID: <87tutpopfj.fsf@notabene.neil.brown.name>
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On Mon, Nov 16 2020, Trond Myklebust wrote:

> On Mon, 2020-11-16 at 16:00 +1100, NeilBrown wrote:
>> On Mon, Nov 16 2020, Trond Myklebust wrote:
>>=20
>> > On Mon, 2020-11-16 at 15:43 +1100, NeilBrown wrote:
>> > > On Mon, Nov 16 2020, Trond Myklebust wrote:
>> > >=20
>> > > > On Mon, 2020-11-16 at 13:59 +1100, NeilBrown wrote:
>> > > > >=20
>> > > > > 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.
>> > > > >=20
>> > > > > 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.
>> > > > >=20
>> > > > > If you:
>> > > > > =C2=A0- mount an NFS filesystem which contains a directory
>> > > > > =C2=A0- mount something (e.g. tmpfs) on that directory
>> > > > > =C2=A0- use iptables (or scissors) to block traffic to the server
>> > > > > =C2=A0- ls -l the-mounted-on-directory
>> > > > > =C2=A0- interrupt the 'ls -l'
>> > > > > you will find that the directory has been unmounted.
>> > > > >=20
>> > > > > This can be fixed by returning the actual error code from
>> > > > > nfs_lookup_verify_inode() rather then zero (except for -
>> > > > > ESTALE).
>> > > > >=20
>> > > > > Fixes: 5ceb9d7fdaaf ("NFS: Refactor nfs_lookup_revalidate()")
>> > > > > Signed-off-by: NeilBrown <neilb@suse.de>
>> > > > > ---
>> > > > > =C2=A0fs/nfs/dir.c | 8 +++++---
>> > > > > =C2=A01 file changed, 5 insertions(+), 3 deletions(-)
>> > > > >=20
>> > > > > 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,
>> > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 unsigned int flags)
>> > > > > =C2=A0{
>> > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0struct inode *in=
ode;
>> > > > > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0int error;
>> > > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0int error =3D 0;
>> > > > > =C2=A0
>> > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0nfs_inc_stats(di=
r, NFSIOS_DENTRYREVALIDATE);
>> > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0inode =3D d_inod=
e(dentry);
>> > > > > @@ -1372,8 +1372,10 @@ nfs_do_lookup_revalidate(struct inode
>> > > > > *dir,
>> > > > > struct dentry *dentry,
>> > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0 nfs_check_verifier(dir, dentry, flags &
>> > > > > LOOKUP_RCU))
>> > > > > {
>> > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0error =3D nfs_lookup_verify_inode(inode,
>> > > > > flags);
>> > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (error) {
>> > > > > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
if (error =3D=3D -ESTALE)
>> > > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
if (error =3D=3D -ESTALE) {
>> > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0nfs_zap_caches(dir);
>> > > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0error =3D 0;
>> > > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0}
>> > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0goto out_bad;
>> > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0}
>> > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0nfs_advise_use_readdirplus(dir);
>> > > > > @@ -1395,7 +1397,7 @@ nfs_do_lookup_revalidate(struct inode
>> > > > > *dir,
>> > > > > struct dentry *dentry,
>> > > > > =C2=A0out_bad:
>> > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (flags & LOOK=
UP_RCU)
>> > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0return -ECHILD;
>> > > > > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0return nfs_lookup_rev=
alidate_done(dir, dentry, inode,
>> > > > > 0);
>> > > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0return nfs_lookup_rev=
alidate_done(dir, dentry, inode,
>> > > > > error);
>> > > >=20
>> > > > 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.
>> > > >=20
>> > > > 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?
>> > >=20
>> > > ERESTARTSYS is the error that easily causes problems.
>> > >=20
>> > > Returning 0 causes d_invalidate() to be called which is quite
>> > > heavy
>> > > handed in mountpoints.
>> >=20
>> > My point is that it shouldn't get returned for mountpoints. See
>> > nfs_lookup_revalidate_done().
>>=20
>> nfs_lookup_revalidate_done() only checks IS_ROOT(), and while many
>> mountpoints are IS_ROOT(), not all are (--bind easily makes others).
>>=20
>> But that isn't even really relevant here.=C2=A0 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.
>

I guess you could do that.
But why would you want to call d_invalidate() just because a signal was
received, or a memory allocation failed?

NeilBrown


>>=20
>> NeilBrown
>>=20
>>=20
>> >=20
>> > > So it is only reasonable to return 0 when we have unambiguous
>> > > confirmation from the server that the object no longer exists.=C2=A0
>> > > ESTALE
>> > > is unambiguous. EIO might be unambiguous.=C2=A0 ERESTARTSYS, ENOMEM,
>> > > ETIMEDOUT are transient and don't justify d_invalidate() being
>> > > called.
>> > >=20
>> > > (BTW, Commit cc89684c9a26 ("NFS: only invalidate dentrys that are
>> > > clearly invalid.")
>> > > =C2=A0fixed much the same bug 3 years ago).
>> > > =C2=A0
>> > > Thanks,
>> > > NeilBrown
>> > >=20
>> > >=20
>> > > >=20
>> > > > > =C2=A0}
>> > > > > =C2=A0
>> > > > > =C2=A0static int
>> > > >=20
>> > > > --=20
>> > > > Trond Myklebust
>> > > > Linux NFS client maintainer, Hammerspace
>> > > > trond.myklebust@hammerspace.com
>> >=20
>> > --=20
>> > Trond Myklebust
>> > CTO, Hammerspace Inc
>> > 4984 El Camino Real, Suite 208
>> > Los Altos, CA 94022
>> > =E2=80=8B
>> > www.hammer.space
>
> --=20
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com

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