On Fri, Nov 06, 2020 at 09:20:04AM +0800, Alex Shi wrote: > From 2fd278b1ca6c3e260ad249808b62f671d8db5a7b Mon Sep 17 00:00:00 2001 > From: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx> > Date: Thu, 5 Nov 2020 11:38:24 +0800 > Subject: [PATCH v21 06/19] mm/rmap: stop store reordering issue on > page->mapping > > Hugh Dickins and Minchan Kim observed a long time issue which > discussed here, but actully the mentioned fix missed. > https://lore.kernel.org/lkml/20150504031722.GA2768@blaptop/ > The store reordering may cause problem in the scenario: > > CPU 0 CPU1 > do_anonymous_page > page_add_new_anon_rmap() > page->mapping = anon_vma + PAGE_MAPPING_ANON > lru_cache_add_inactive_or_unevictable() > spin_lock(lruvec->lock) > SetPageLRU() > spin_unlock(lruvec->lock) > /* idletacking judged it as LRU > * page so pass the page in > * page_idle_clear_pte_refs > */ > page_idle_clear_pte_refs > rmap_walk > if PageAnon(page) > > Johannes give detailed examples how the store reordering could cause > a trouble: > "The concern is the SetPageLRU may get reorder before 'page->mapping' > setting, That would make CPU 1 will observe at page->mapping after > observing PageLRU set on the page. > > 1. anon_vma + PAGE_MAPPING_ANON > > That's the in-order scenario and is fine. > > 2. NULL > > That's possible if the page->mapping store gets reordered to occur > after SetPageLRU. That's fine too because we check for it. > > 3. anon_vma without the PAGE_MAPPING_ANON bit > > That would be a problem and could lead to all kinds of undesirable > behavior including crashes and data corruption. > > Is it possible? AFAICT the compiler is allowed to tear the store to > page->mapping and I don't see anything that would prevent it. > > That said, I also don't see how the reader testing PageLRU under the > lru_lock would prevent that in the first place. AFAICT we need that > WRITE_ONCE() around the page->mapping assignment." > > Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: Hugh Dickins <hughd@xxxxxxxxxx> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> > Cc: Minchan Kim <minchan@xxxxxxxxxx> > Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx> > Cc: linux-kernel@xxxxxxxxxxxxxxx > Cc: linux-mm@xxxxxxxxx Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> Thanks Alex!
