Hello Andrew, On Tue, May 12, 2015 at 03:28:40PM -0700, Andrew Morton wrote: > On Tue, 12 May 2015 13:18:39 +0300 Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> 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. > > > > ... > > > > --- 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); > > Please let's not put things like WRITE_ONCE() in there without > documenting them - otherwise it's terribly hard for readers to work out > why it was added. > > How's this look? > > --- a/mm/rmap.c~rmap-fix-theoretical-race-between-do_wp_page-and-shrink_active_list-fix > +++ a/mm/rmap.c > @@ -950,6 +950,11 @@ void page_move_anon_rmap(struct page *pa > VM_BUG_ON_PAGE(page->index != linear_page_index(vma, address), page); > > anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON; > + /* > + * Ensure that anon_vma and the PAGE_MAPPING_ANON bit are written > + * simultaneously, so a concurrent reader (eg shrink_active_list) will IMHO, rather than shrink_active_list, PageAnon in page_referenced is better to me. -- Kind regards, Minchan Kim -- 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>