On Mon, 2024-09-09 at 10:17 -0400, Olga Kornievskaia wrote:
> On Mon, Sep 9, 2024 at 8:24 AM Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> >
> > On Mon, 2024-09-09 at 15:06 +1000, NeilBrown wrote:
> > > The pair of bloom filtered used by delegation_blocked() was intended to
> > > block delegations on given filehandles for between 30 and 60 seconds. A
> > > new filehandle would be recorded in the "new" bit set. That would then
> > > be switch to the "old" bit set between 0 and 30 seconds later, and it
> > > would remain as the "old" bit set for 30 seconds.
> > >
> >
> > Since we're on the subject...
> >
> > 60s seems like an awfully long time to block delegations on an inode.
> > Recalls generally don't take more than a few seconds when things are
> > functioning properly.
> >
> > Should we swap the bloom filters more often?
>
> I was also thinking that perhaps we can do 15-30s perhaps? Another
> thought was should this be a configurable value (with some
> non-negotiable min and max)?
>
Even 15-30s sounds like a lot, but that would probably be better. While
I'm not a huge fan of tunables, it's hard to know what the "right"
interval is here without some experimentation.
Maybe we should start with some metrics. For example, how often does
the bloom filter block a delegation vs. allow it to pass? That kind of
thing would be good to know.
> > > Unfortunately the code intended to clear the old bit set once it reached
> > > 30 seconds old, preparing it to be the next new bit set, instead cleared
> > > the *new* bit set before switching it to be the old bit set. This means
> > > that the "old" bit set is always empty and delegations are blocked
> > > between 0 and 30 seconds.
> > >
> > > This patch updates bd->new before clearing the set with that index,
> > > instead of afterwards.
> > >
> > > Reported-by: Olga Kornievskaia <okorniev@xxxxxxxxxx>
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Fixes: 6282cd565553 ("NFSD: Don't hand out delegations for 30 seconds after recalling them.")
> > > Signed-off-by: NeilBrown <neilb@xxxxxxx>
> > > ---
> > > fs/nfsd/nfs4state.c | 5 +++--
> > > 1 file changed, 3 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> > > index 4313addbe756..6f18c1a7af2e 100644
> > > --- a/fs/nfsd/nfs4state.c
> > > +++ b/fs/nfsd/nfs4state.c
> > > @@ -1078,7 +1078,8 @@ static void nfs4_free_deleg(struct nfs4_stid *stid)
> > > * When a delegation is recalled, the filehandle is stored in the "new"
> > > * filter.
> > > * Every 30 seconds we swap the filters and clear the "new" one,
> > > - * unless both are empty of course.
> > > + * unless both are empty of course. This results in delegations for a
> > > + * given filehandle being blocked for between 30 and 60 seconds.
> > > *
> > > * Each filter is 256 bits. We hash the filehandle to 32bit and use the
> > > * low 3 bytes as hash-table indices.
> > > @@ -1107,9 +1108,9 @@ static int delegation_blocked(struct knfsd_fh *fh)
> > > if (ktime_get_seconds() - bd->swap_time > 30) {
> > > bd->entries -= bd->old_entries;
> > > bd->old_entries = bd->entries;
> > > + bd->new = 1-bd->new;
> > > memset(bd->set[bd->new], 0,
> > > sizeof(bd->set[0]));
> > > - bd->new = 1-bd->new;
> > > bd->swap_time = ktime_get_seconds();
> > > }
> > > spin_unlock(&blocked_delegations_lock);
> >
> > --
> > Jeff Layton <jlayton@xxxxxxxxxx>
> >
>
--
Jeff Layton <jlayton@xxxxxxxxxx>
