From 16f5e707d6f6f7644ff07e583b8f18c3dcc5499f Mon Sep 17 00:00:00 2001 From: Alex Shi Date: Tue, 15 Dec 2020 12:33:42 -0800 Subject: [PATCH] mm/rmap: stop store reordering issue on page->mapping MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Hugh Dickins and Minchan Kim observed a long time issue which discussed here, but actully the mentioned fix in https://lore.kernel.org/lkml/20150504031722.GA2768@blaptop/ was missed. 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 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." [alex.shi@linux.alibaba.com: updated for comments change from Johannes] Link: https://lkml.kernel.org/r/e66ef2e5-c74c-6498-e8b3-56c37b9d2d15@linux.alibaba.com Link: https://lkml.kernel.org/r/1604566549-62481-7-git-send-email-alex.shi@linux.alibaba.com Signed-off-by: Alex Shi Acked-by: Johannes Weiner Acked-by: Hugh Dickins Cc: Matthew Wilcox Cc: Minchan Kim Cc: Vladimir Davydov Cc: Alexander Duyck Cc: Andrea Arcangeli Cc: Andrey Ryabinin Cc: "Chen, Rong A" Cc: Daniel Jordan Cc: "Huang, Ying" Cc: Jann Horn Cc: Joonsoo Kim Cc: Kirill A. Shutemov Cc: Kirill A. Shutemov Cc: Konstantin Khlebnikov Cc: Mel Gorman Cc: Michal Hocko Cc: Michal Hocko Cc: Mika Penttilä Cc: Shakeel Butt Cc: Tejun Heo Cc: Thomas Gleixner Cc: Vlastimil Babka Cc: Wei Yang Cc: Yang Shi Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/rmap.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/mm/rmap.c b/mm/rmap.c index 6657000b18d4..ab16c96efdfb 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1054,8 +1054,14 @@ static void __page_set_anon_rmap(struct page *page, if (!exclusive) anon_vma = anon_vma->root; + /* + * page_idle does a lockless/optimistic rmap scan on page->mapping. + * Make sure the compiler doesn't split the stores of anon_vma and + * the PAGE_MAPPING_ANON type identifier, otherwise the rmap code + * could mistake the mapping for a struct address_space and crash. + */ anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON; - page->mapping = (struct address_space *) anon_vma; + WRITE_ONCE(page->mapping, (struct address_space *) anon_vma); page->index = linear_page_index(vma, address); }