在 2019/11/23 上午12:16, Johannes Weiner 写道: > On Fri, Nov 22, 2019 at 10:36:32AM +0800, Alex Shi wrote: >> 在 2019/11/22 上午6:06, Johannes Weiner 写道: >>> If we could restrict lock_page_lruvec() to working only on PageLRU >>> pages, we could fix the problem with memory barriers. But this won't >>> work for split_huge_page(), which is AFAICT the only user that needs >>> to freeze the lru state of a page that could be isolated elsewhere. >>> >>> So AFAICS the only option is to lock out mem_cgroup_move_account() >>> entirely when the lru_lock is held. Which I guess should be fine. >> >> I guess we can try from lock_page_memcg, is that a good start? > > Yes. > >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >> index 7e6387ad01f0..f4bbbf72c5b8 100644 >> --- a/mm/memcontrol.c >> +++ b/mm/memcontrol.c >> @@ -1224,7 +1224,7 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgd >> goto out; >> } >> >> - memcg = page->mem_cgroup; >> + memcg = lock_page_memcg(page); >> /* >> * Swapcache readahead pages are added to the LRU - and >> * possibly migrated - before they are charged. > > test_clear_page_writeback() calls this function with that lock already > held so that would deadlock. Let's keep locking in lock_page_lruvec(). > > lock_page_lruvec(): > > memcg = lock_page_memcg(page); > lruvec = mem_cgroup_lruvec(page_pgdat(page), memcg); > > spin_lock_irqsave(&lruvec->lru_lock, *flags); > return lruvec; > > unlock_lruvec(); > > spin_unlock_irqrestore(&lruvec->lru_lock); > __unlock_page_memcg(lruvec_memcg(lruvec)); > > The lock ordering should be fine as well. But it might be a good idea > to stick a might_lock(&memcg->move_lock) in lock_page_memcg() before > that atomic_read() and test with lockdep enabled. Hi Johannes, Thanks a lot for the suggestion. I will look into this and try. > > > But that leaves me with one more worry: compaction. We locked out > charge moving now, so between that and knowing that the page is alive, > we have page->mem_cgroup stable. But compaction doesn't know whether > the page is alive - it comes from a pfn and finds out using PageLRU. > > In the current code, pgdat->lru_lock remains the same before and after > the page is charged to a cgroup, so once compaction has that locked > and it observes PageLRU, it can go ahead and isolate the page. > > But lruvec->lru_lock changes during charging, and then compaction may > hold the wrong lock during isolation: > > compaction: generic_file_buffered_read: > > page_cache_alloc() > > !PageBuddy() > > lock_page_lruvec(page) > lruvec = mem_cgroup_page_lruvec() > spin_lock(&lruvec->lru_lock) > if lruvec != mem_cgroup_page_lruvec() > goto again > > add_to_page_cache_lru() > mem_cgroup_commit_charge() > page->mem_cgroup = foo > lru_cache_add() > __pagevec_lru_add() > SetPageLRU() > > if PageLRU(page): > __isolate_lru_page() > > I don't see what prevents the lruvec from changing under compaction, > neither in your patches nor in Hugh's. Maybe I'm missing something? Yes, it's a problem. Guess we could move the lruvec recheck after PageLRU() test in compaction. Then it could be safe, and add a bit more burden on compaction should be fine. at last we have no disturb to file read. Thanks Alex
