On 05/12/2015 09:43 PM, Minchan Kim wrote: > Hi, Rik > > I'd like to bring up the issue in this thread although I already gave > my Acked-by. > > Below issue causes by no PG_locked page in page_referenced while > page_move_anon_rmap depends on PG_locked to prevent race with rmap code. > > So, although this patch fixes below one example, we still have a problem > in rmap. > > If page_referenced holds PG_locked for all of pages unconditionally, > we don't need this patch and might remove READ_ONCE introduced by > 80e148 and more than. > > What do you think about? Maybe the reclaim code and page_referenced are fine. However, I have seen one real world bug report of a page->mapping pointing to an anon_vma without the PAGE_MAPPING_ANON bit being set. This is a pretty hard to hit race, so I have only ever heard of it happening once, and I do not remember the details of exactly what code blew up trying to follow the page->mapping pointer in the wrong way. I wish I remember what needs this patch, but I have a rather strong suspicion there is something that needs it... Acked-by: Rik van Riel <riel@xxxxxxxxxx> > On Tue, May 12, 2015 at 01:18:39PM +0300, Vladimir Davydov wrote: >> As noted by Paul the compiler is free to store a temporary result in a >> variable on stack, heap or global unless it is explicitly marked as >> volatile, see: >> >> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4455.html#sample-optimizations >> >> This can result in a race between do_wp_page() and shrink_active_list() >> as follows. >> >> In do_wp_page() we can call page_move_anon_rmap(), which sets >> page->mapping as follows: >> >> anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON; >> page->mapping = (struct address_space *) anon_vma; >> >> The page in question may be on an LRU list, because nowhere in >> do_wp_page() we remove it from the list, neither do we take any LRU >> related locks. Although the page is locked, shrink_active_list() can >> still call page_referenced() on it concurrently, because the latter does >> not require an anonymous page to be locked: >> >> CPU0 CPU1 >> ---- ---- >> do_wp_page shrink_active_list >> lock_page page_referenced >> PageAnon->yes, so skip trylock_page >> page_move_anon_rmap >> page->mapping = anon_vma >> rmap_walk >> PageAnon->no >> rmap_walk_file >> BUG >> page->mapping += PAGE_MAPPING_ANON >> >> This patch fixes this race by explicitly forbidding the compiler to >> split page->mapping store in page_move_anon_rmap() with the aid of >> WRITE_ONCE. >> >> Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> >> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> >> Cc: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> >> Cc: Rik van Riel <riel@xxxxxxxxxx> >> Cc: Hugh Dickins <hughd@xxxxxxxxxx> >> --- >> Changes in v2: >> - do not add READ_ONCE to PageAnon and WRITE_ONCE to >> __page_set_anon_rmap and __hugepage_set_anon_rmap (Kirill) >> >> mm/rmap.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/mm/rmap.c b/mm/rmap.c >> index 24dd3f9fee27..8b18fd4227d1 100644 >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -950,7 +950,7 @@ void page_move_anon_rmap(struct page *page, >> VM_BUG_ON_PAGE(page->index != linear_page_index(vma, address), page); >> >> anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON; >> - page->mapping = (struct address_space *) anon_vma; >> + WRITE_ONCE(page->mapping, (struct address_space *) anon_vma); >> } >> >> /** >> -- >> 1.7.10.4 >> >> -- >> To unsubscribe, send a message with 'unsubscribe linux-mm' in >> the body to majordomo@xxxxxxxxx. For more info on Linux MM, >> see: http://www.linux-mm.org/ . >> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> > -- All rights reversed -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>