Messages in this thread Patch in this message | | | From | Vladimir Davydov <> | Subject | [PATCH v2] rmap: fix theoretical race between do_wp_page and shrink_active_list | Date | Tue, 12 May 2015 13:18:39 +0300 |
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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@parallels.com> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Cc: "Kirill A. Shutemov" <kirill@shutemov.name> Cc: Rik van Riel <riel@redhat.com> Cc: Hugh Dickins <hughd@google.com> --- 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
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