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 --- mm/rmap.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/mm/rmap.c b/mm/rmap.c index 1b84945d655c..078d54da59d4 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1054,8 +1054,13 @@ static void __page_set_anon_rmap(struct page *page, if (!exclusive) anon_vma = anon_vma->root; + /* + * Prevent page->mapping from pointing to an anon_vma without + * the PAGE_MAPPING_ANON bit set. This could happen if the + * compiler stores anon_vma and then adds PAGE_MAPPING_ANON to it. + */ 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); } -- 1.8.3.1
