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

[Alex Shi]

å 2020/4/15 äå12:31, Johannes Weiner åé:
> On Tue, Apr 14, 2020 at 12:52:30PM +0800, Alex Shi wrote:
>> å 2020/4/14 äå2:07, Johannes Weiner åé:
>>> Plus, the overhead of tracking is tiny - 512k per G of swap (0.04%).
>>>
>>> Maybe we should just delete MEMCG_SWAP and unconditionally track swap
>>> entry ownership when the memory controller is enabled. I don't see a
>>> good reason not to, and it would simplify the entire swapin path, the
>>> LRU locking, and the page->mem_cgroup stabilization rules.
>>>
>>
>> Sorry for not follow you up, did you mean just remove the MEMCG_SWAP configuration
>> and keep the feature in default memcg? 
> 
> Yes.
> 
>> That does can remove lrucare, but PageLRU lock scheme still fails since
>> we can not isolate the page during commit_charge, is that right?
> 
> No, without lrucare the scheme works. Charges usually do:
> 
> page->mem_cgroup = new;
> SetPageLRU(page);
> 
> And so if you can TestClearPageLRU(), page->mem_cgroup is stable.
> 
> lrucare charging is the exception: it changes page->mem_cgroup AFTER
> PageLRU has already been set, and even when it CANNOT acquire the
> PageLRU lock itself. It violates the rules.
> 
> If we make MEMCG_SWAP mandatory, we always have cgroup records for
> swapped out pages. That means we can charge all swapin pages
> (incl. readahead pages) directly in __read_swap_cache_async(), before
> setting PageLRU on the new pages.
> 
> Then we can delete lrucare.
> 
> And then TestClearPageLRU() guarantees page->mem_cgroup is stable.
> 

Hi Johannes,

Thanks a lot for point out!

Charging in __read_swap_cache_async would ask for 3 layers function arguments
pass, that would be a bit ugly. Compare to this, could we move out the
lru_cache add after commit_charge, like ksm copied pages?

That give a bit extra non lru list time, but the page just only be used only
after add_anon_rmap setting. Could it cause troubles?

I tried to track down the reason of lru_cache_add here, but no explanation
till first git kernel commit.

Thanks
Alex