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

[Alberto Garcia]

On Mon 06 Aug 2018 05:58:41 PM CEST, Kevin Wolf wrote:
> 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.

When we're waiting for a cache entry to flush and the coroutine yields,
can the previous (already checked) cache entries become dirty?

Berto