在 2020/4/15 上午12:31, Johannes Weiner 写道: > 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. > 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
