Re: [Qemu-devel] [PATCH] qcow2: Release dirty entries with cache-clean-interval

[Kevin Wolf <kwolf@redhat.com>]

Am 06.08.2018 um 17:20 hat Alberto Garcia geschrieben:
> On Mon 06 Aug 2018 05:05:41 PM CEST, Kevin Wolf wrote:
> > Am 06.08.2018 um 16:13 hat Alberto Garcia geschrieben:
> >> -static inline bool can_clean_entry(Qcow2Cache *c, int i)
> >> +static inline bool can_clean_entry(BlockDriverState *bs, Qcow2Cache *c, int i)
> >>  {
> >>      Qcow2CachedTable *t = &c->entries[i];
> >> -    return t->ref == 0 && !t->dirty && t->offset != 0 &&
> >> -        t->lru_counter <= c->cache_clean_lru_counter;
> >> +    if (t->ref || !t->offset || t->lru_counter > c->cache_clean_lru_counter) {
> >> +        return false;
> >> +    }
> >> +
> >> +    if (qcow2_cache_entry_flush(bs, c, i) < 0) {
> >> +        return false;
> >> +    }
> >
> > We're not in coroutine context here, so qcow2_cache_entry_flush() will
> > be blocking. I don't think that's acceptable in a timer callback.
> >
> > On the other hand, if we made it non-blocking by moving it into a
> > coroutine that could yield, we would have to consider races with other
> > parts of the code and at least take s->lock and implement
> > .bdrv_co_drain_begin/end callbacks.
> 
> Oh, I see... it's probably not worth complicating the code for this then.

On second thoughts, I actually think we can do without the drain
callbacks if we just add a bdrv_inc/dec_in_flight() pair around
qcow2_cache_clean_unused().

We'd still have to create a coroutine in cache_clean_timer_cb() and take
the lock, but that sounds more managable.

Kevin