Re: [PATCH v8 03/10] mm/lru: replace pgdat lru_lock with lruvec lock

[Alex Shi]

å 2020/1/22 äå12:00, Johannes Weiner åé:
> On Mon, Jan 20, 2020 at 08:58:09PM +0800, Alex Shi wrote:
>>
>>
>> å 2020/1/17 äå5:52, Johannes Weiner åé:
>>
>>> You simply cannot serialize on page->mem_cgroup->lruvec when
>>> page->mem_cgroup isn't stable. You need to serialize on the page
>>> itself, one way or another, to make this work.
>>>
>>>
>>> So here is a crazy idea that may be worth exploring:
>>>
>>> Right now, pgdat->lru_lock protects both PageLRU *and* the lruvec's
>>> linked list.
>>>
>>> Can we make PageLRU atomic and use it to stabilize the lru_lock
>>> instead, and then use the lru_lock only serialize list operations?
>>>
>>
>> Hi Johannes,
>>
>> I am trying to figure out the solution of atomic PageLRU, but is 
>> blocked by the following sitations, when PageLRU and lru list was protected
>> together under lru_lock, the PageLRU could be a indicator if page on lru list
>> But now seems it can't be the indicator anymore.
>> Could you give more clues of stabilization usage of PageLRU?
> 
> There are two types of PageLRU checks: optimistic and deterministic.
> 
> The check in activate_page() for example is optimistic and the result
> unstable, but that's okay, because if we miss a page here and there
> it's not the end of the world.
> 
> But the check in __activate_page() is deterministic, because we need
> to be sure before del_page_from_lru_list(). Currently it's made
> deterministic by testing under the lock: whoever acquires the lock
> first gets to touch the LRU state. The same can be done with an atomic
> TestClearPagLRU: whoever clears the flag first gets to touch the LRU
> state (the lock is then only acquired to not corrupt the linked list,
> in case somebody adds or removes a different page at the same time).

Hi Johannes,

Thanks a lot for detailed explanations! I just gonna to take 2 weeks holiday
from tomorrow as Chinese new year season with families. I am very sorry for 
can not hang on this for a while.

> 
> I.e. in my proposal, if you want to get a stable read of PageLRU, you
> have to clear it atomically. But AFAICS, everybody who currently does
> need a stable read either already clears it or can easily be converted
> to clear it and then set it again (like __activate_page and friends).
> 
>> __page_cache_release/release_pages/compaction            __pagevec_lru_add
>> if (TestClearPageLRU(page))                              if (!PageLRU())
>>                                                                 lruvec_lock();
>>                                                                 list_add();
>>         			                                lruvec_unlock();
>>         			                                SetPageLRU() //position 1
>>         lock_page_lruvec_irqsave(page, &flags);
>>         del_page_from_lru_list(page, lruvec, ..);
>>         unlock_page_lruvec_irqrestore(lruvec, flags);
>>                                                                 SetPageLRU() //position 2
> 
> Hm, that's not how __pagevec_lru_add() looks. In fact,
> __pagevec_lru_add_fn() has a BUG_ON(PageLRU).
> 
> That's because only one thread can own the isolation state at a time.
> 
> If PageLRU is set, only one thread can claim it. Right now, whoever
> takes the lock first and clears it wins. When we replace it with
> TestClearPageLRU, it's the same thing: only one thread can win.
> 
> And you cannot set PageLRU, unless you own it. Either you isolated the
> page using TestClearPageLRU, or you allocated a new page.

Yes I understand isolatation would exclusive by PageLRU, but forgive my
stupid, I didn't figure out how a new page lruvec adding could be blocked.

Anyway, I will try my best to catch up after holiday.

Many thanks for nice cocaching!
Alex