On Thu 29-06-23 19:47:08, Baokun Li wrote:
> On 2023/6/29 18:59, Jan Kara wrote:
> > On Wed 28-06-23 21:21:53, Baokun Li wrote:
> > > @@ -760,6 +771,8 @@ dqcache_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> > > struct dquot *dquot;
> > > unsigned long freed = 0;
> > > + flush_delayed_work("a_release_work);
> > > +
> > I would not flush the work here. Sure, it can make more dquots available
> > for reclaim but I think it is more important for the shrinker to not wait
> > on srcu period as shrinker can be called very frequently under memory
> > pressure.
> This is because I want to use remove_free_dquot() directly, and if I don't
> do
> flush here anymore, then DQST_FREE_DQUOTS will not be accurate.
> Since that's the case, I'll remove the flush here and add a determination
> to remove_free_dquot() whether to increase DQST_FREE_DQUOTS.
OK.
> > > spin_lock(&dq_list_lock);
> > > while (!list_empty(&free_dquots) && sc->nr_to_scan) {
> > > dquot = list_first_entry(&free_dquots, struct dquot, dq_free);
> > > @@ -787,6 +800,60 @@ static struct shrinker dqcache_shrinker = {
> > > .seeks = DEFAULT_SEEKS,
> > > };
> > > +/*
> > > + * Safely release dquot and put reference to dquot.
> > > + */
> > > +static void quota_release_workfn(struct work_struct *work)
> > > +{
> > > + struct dquot *dquot;
> > > + struct list_head rls_head;
> > > +
> > > + spin_lock(&dq_list_lock);
> > > + /* Exchange the list head to avoid livelock. */
> > > + list_replace_init(&releasing_dquots, &rls_head);
> > > + spin_unlock(&dq_list_lock);
> > > +
> > > +restart:
> > > + synchronize_srcu(&dquot_srcu);
> > > + spin_lock(&dq_list_lock);
> > > + while (!list_empty(&rls_head)) {
> > I think the logic below needs a bit more work. Firstly, I think that
> > dqget() should removing dquots from releasing_dquots list - basically just
> > replace the:
> > if (!atomic_read(&dquot->dq_count))
> > remove_free_dquot(dquot);
> > with
> > /* Dquot on releasing_dquots list? Drop ref kept by that list. */
> > if (atomic_read(&dquot->dq_count) == 1 && !list_empty(&dquot->dq_free))
> > atomic_dec(&dquot->dq_count);
> > remove_free_dquot(dquot);
> > atomic_inc(&dquot->dq_count);
> >
> > That way we are sure that while we are holding dq_list_lock, all dquots on
> > rls_head list have dq_count == 1.
> I wrote it this way at first, but that would have been problematic, so I
> ended up dropping the dq_count == 1 constraint for dquots on
> releasing_dquots. Like the following, we will get a bad dquot directly:
>
> quota_release_workfn
> spin_lock(&dq_list_lock)
> dquot = list_first_entry(&rls_head, struct dquot, dq_free)
> spin_unlock(&dq_list_lock)
> dquot->dq_sb->dq_op->release_dquot(dquot)
> release_dquot
> dqget
> atomic_dec(&dquot->dq_count)
> remove_free_dquot(dquot)
> atomic_inc(&dquot->dq_count)
> spin_unlock(&dq_list_lock)
> wait_on_dquot(dquot)
> if (!test_bit(DQ_ACTIVE_B, &dquot->dq_flags))
> // still active
> mutex_lock(&dquot->dq_lock)
> dquot_is_busy(dquot)
> atomic_read(&dquot->dq_count) > 1
> clear_bit(DQ_ACTIVE_B, &dquot->dq_flags)
> mutex_unlock(&dquot->dq_lock)
>
> Removing dquot from releasing_dquots and its reduced reference count
> will cause dquot_is_busy() in dquot_release to fail. wait_on_dquot(dquot)
> in dqget would have no effect. This is also the reason why I did not restart
> at dquot_active. Adding dquot to releasing_dquots only in dqput() and
> removing dquot from releasing_dquots only in quota_release_workfn() is
> a simple and effective way to ensure consistency.
Indeed, that's a good point. Still cannot we simplify the loop like:
while (!list_empty(&rls_head)) {
dquot = list_first_entry(&rls_head, struct dquot, dq_free);
/* Dquot got used again? */
if (atomic_read(&dquot->dq_count) > 1) {
atomic_dec(&dquot->dq_count);
remove_free_dquot(dquot);
continue;
}
if (dquot_dirty(dquot)) {
keep what you had
}
if (dquot_active(dquot)) {
spin_unlock(&dq_list_lock);
dquot->dq_sb->dq_op->release_dquot(dquot);
goto restart;
}
/* Dquot is inactive and clean, we can move it to free list */
atomic_dec(&dquot->dq_count);
remove_free_dquot(dquot);
put_dquot_last(dquot);
}
What do you think?
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
