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

[Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>]

On Tue, Apr 14, 2020 at 12:52:30PM +0800, Alex Shi wrote:
> 在 2020/4/14 上午2:07, Johannes Weiner 写道:
> > 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.