From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx105.postini.com [74.125.245.105]) by kanga.kvack.org (Postfix) with SMTP id 374646B0034 for ; Mon, 15 Jul 2013 16:43:57 -0400 (EDT) Received: by mail-pb0-f41.google.com with SMTP id rp16so11754429pbb.28 for ; Mon, 15 Jul 2013 13:43:56 -0700 (PDT) Date: Tue, 16 Jul 2013 04:43:54 +0800 From: Shaohua Li Subject: [patch 3/4 v6]swap: fix races exposed by swap discard Message-ID: <20130715204354.GC7925@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Sender: owner-linux-mm@kvack.org List-ID: To: linux-mm@kvack.org Cc: akpm@linux-foundation.org, riel@redhat.com, minchan@kernel.org, kmpark@infradead.org, hughd@google.com, aquini@redhat.com Last patch can expose races, according to Hugh: swapoff was sometimes failing with "Cannot allocate memory", coming from try_to_unuse()'s -ENOMEM: it needs to allow for swap_duplicate() failing on a free entry temporarily SWAP_MAP_BAD while being discarded. We should use ACCESS_ONCE() there, and whenever accessing swap_map locklessly; but rather than peppering it throughout try_to_unuse(), just declare *swap_map with volatile. try_to_unuse() is accustomed to *swap_map going down racily, but not necessarily to it jumping up from 0 to SWAP_MAP_BAD: we'll be safer to prevent that transition once SWP_WRITEOK is switched off, when it's a waste of time to issue discards anyway (swapon can do a whole discard). Another issue is: In swapin_readahead(), read_swap_cache_async() can read a bad swap entry, because we don't check if readahead swap entry is bad. This doesn't break anything but such swapin page is wasteful and can only be freed at page reclaim. We should avoid read such swap entry. And in discard, we mark swap entry SWAP_MAP_BAD and then switch it to normal when discard is finished. If readahead reads such swap entry, we have the same issue, so we much check if swap entry is bad too. Thanks Hugh to inspire swapin_readahead could use bad swap entry. [include Hugh's patch 'swap: fix swapoff ENOMEMs from discard'] Signed-off-by: Hugh Dickins Signed-off-by: Shaohua Li --- mm/swapfile.c | 15 +++++++++++---- 1 file changed, 11 insertions(+), 4 deletions(-) Index: linux/mm/swapfile.c =================================================================== --- linux.orig/mm/swapfile.c 2013-07-11 19:14:44.121818963 +0800 +++ linux/mm/swapfile.c 2013-07-11 19:14:46.953783361 +0800 @@ -357,7 +357,8 @@ static void dec_cluster_info_page(struct * instead of free it immediately. The cluster will be freed * after discard. */ - if (p->flags & SWP_PAGE_DISCARD) { + if ((p->flags & (SWP_WRITEOK | SWP_PAGE_DISCARD)) == + (SWP_WRITEOK | SWP_PAGE_DISCARD)) { swap_cluster_schedule_discard(p, idx); return; } @@ -1255,7 +1256,7 @@ static unsigned int find_next_to_unuse(s else continue; } - count = si->swap_map[i]; + count = ACCESS_ONCE(si->swap_map[i]); if (count && swap_count(count) != SWAP_MAP_BAD) break; } @@ -1275,7 +1276,7 @@ int try_to_unuse(unsigned int type, bool { struct swap_info_struct *si = swap_info[type]; struct mm_struct *start_mm; - unsigned char *swap_map; + volatile unsigned char *swap_map; /* ACCESS_ONCE throughout */ unsigned char swcount; struct page *page; swp_entry_t entry; @@ -1326,7 +1327,8 @@ int try_to_unuse(unsigned int type, bool * reused since sys_swapoff() already disabled * allocation from here, or alloc_page() failed. */ - if (!*swap_map) + swcount = *swap_map; + if (!swcount || swcount == SWAP_MAP_BAD) continue; retval = -ENOMEM; break; @@ -2506,6 +2508,11 @@ static int __swap_duplicate(swp_entry_t goto unlock_out; count = p->swap_map[offset]; + if (unlikely(swap_count(count) == SWAP_MAP_BAD)) { + err = -ENOENT; + goto unlock_out; + } + has_cache = count & SWAP_HAS_CACHE; count &= ~SWAP_HAS_CACHE; err = 0; -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org