Subject: + swap-fix-races-exposed-by-swap-discard.patch added to -mm tree To: shli@xxxxxxxxxx,aquini@xxxxxxxxxx,hughd@xxxxxxxxxx,kmpark@xxxxxxxxxxxxx,minchan@xxxxxxxxxx,riel@xxxxxxxxxx,shli@xxxxxxxxxxxx From: akpm@xxxxxxxxxxxxxxxxxxxx Date: Wed, 24 Jul 2013 14:36:08 -0700 The patch titled Subject: swap: fix races exposed by swap discard has been added to the -mm tree. Its filename is swap-fix-races-exposed-by-swap-discard.patch This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/swap-fix-races-exposed-by-swap-discard.patch and later at http://ozlabs.org/~akpm/mmotm/broken-out/swap-fix-races-exposed-by-swap-discard.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/SubmitChecklist when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Shaohua Li <shli@xxxxxxxxxx> Subject: swap: fix races exposed by swap discard 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: Shaohua Li <shli@xxxxxxxxxxxx> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxx> Cc: Minchan Kim <minchan@xxxxxxxxxx> Cc: Kyungmin Park <kmpark@xxxxxxxxxxxxx> Cc: Rafael Aquini <aquini@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/swapfile.c | 31 +++++++++++++++++++++++++++---- 1 file changed, 27 insertions(+), 4 deletions(-) diff -puN mm/swapfile.c~swap-fix-races-exposed-by-swap-discard mm/swapfile.c --- a/mm/swapfile.c~swap-fix-races-exposed-by-swap-discard +++ a/mm/swapfile.c @@ -370,7 +370,8 @@ 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; } @@ -1273,7 +1274,7 @@ 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; } @@ -1293,7 +1294,11 @@ 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; /* swap_map is accessed without + * locking. Mark it as volatile + * to prevent compiler doing + * something odd. + */ unsigned char swcount; struct page *page; swp_entry_t entry; @@ -1344,7 +1349,15 @@ 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; + /* + * We don't hold lock here, so the swap entry could be + * SWAP_MAP_BAD (when the cluster is discarding). + * Instead of fail out, We can just skip the swap + * entry because swapoff will wait for discarding + * finish anyway. + */ + if (!swcount || swcount == SWAP_MAP_BAD) continue; retval = -ENOMEM; break; @@ -2528,6 +2541,16 @@ static int __swap_duplicate(swp_entry_t goto unlock_out; count = p->swap_map[offset]; + + /* + * swapin_readahead() doesn't check if a swap entry is valid, so the + * swap entry could be SWAP_MAP_BAD. Check here with lock held. + */ + 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; _ Patches currently in -mm which might be from shli@xxxxxxxxxx are swap-change-block-allocation-algorithm-for-ssd.patch swap-make-swap-discard-async.patch swap-fix-races-exposed-by-swap-discard.patch swap-make-cluster-allocation-per-cpu.patch swap-add-a-simple-detector-for-inappropriate-swapin-readahead.patch swap-add-a-simple-detector-for-inappropriate-swapin-readahead-fix.patch linux-next.patch -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html