During developemnt for zram-swap asynchronous writeback, I found strange corruption of compressed page. Modules linked in: zram(E) CPU: 3 PID: 1520 Comm: zramd-1 Tainted: G E 4.8.0-mm1-00320-ge0d4894c9c38-dirty #3274 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014 task: ffff88007620b840 task.stack: ffff880078090000 RIP: 0010:[<ffffffff811d6f3d>] [<ffffffff811d6f3d>] set_freeobj.part.43+0x1c/0x1f RSP: 0018:ffff880078093ca8 EFLAGS: 00010246 RAX: 0000000000000018 RBX: ffff880076798d88 RCX: ffffffff81c408c8 RDX: 0000000000000018 RSI: 0000000000000000 RDI: 0000000000000246 RBP: ffff880078093cb0 R08: 0000000000000000 R09: 0000000000000000 R10: ffff88005bc43030 R11: 0000000000001df3 R12: ffff880076798d88 R13: 000000000005bc43 R14: ffff88007819d1b8 R15: 0000000000000001 FS: 0000000000000000(0000) GS:ffff88007e380000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007fc934048f20 CR3: 0000000077b01000 CR4: 00000000000406e0 Stack: ffffea00016f10c0 ffff880078093d08 ffffffff811d43cb ffff88005bc43030 ffff88005bc43030 0000000000000001 ffff88007aa17f68 ffff88007819d1b8 000000000200020a 0000000002000200 ffff880076798d88 ffff88007aa17f68 Call Trace: [<ffffffff811d43cb>] obj_malloc+0x22b/0x260 [<ffffffff811d4be4>] zs_malloc+0x1e4/0x580 [<ffffffff81355490>] ? lz4_compress_crypto+0x30/0x50 [<ffffffffa000269d>] zram_bvec_rw+0x4cd/0x830 [zram] [<ffffffffa000356c>] page_requests_rw+0x9c/0x130 [zram] [<ffffffffa0003600>] ? page_requests_rw+0x130/0x130 [zram] [<ffffffffa00036e6>] zram_thread+0xe6/0x173 [zram] [<ffffffff810b67e0>] ? wake_atomic_t_function+0x60/0x60 [<ffffffff81094cfa>] kthread+0xca/0xe0 [<ffffffff81094c30>] ? kthread_park+0x60/0x60 [<ffffffff817ae775>] ret_from_fork+0x25/0x30 With investigation, it reveals currently stable page doesn't support anonymous page. IOW, reuse_swap_page can reuse the page without waiting writeback completion so it can overwrite page zram is compressing. Unfortunately, zram has used per-cpu stream feature from v4.7. It aims for increasing cache hit ratio of scratch buffer for compressing. Downside of that approach is that zram should ask memory space for compressed page in per-cpu context which requires stricted gfp flag which could be failed. If so, it retries to allocate memory space out of per-cpu context so it could get memory this time and compress the data again, copies it to the memory space. In this scenario, zram assumes the data should never be changed but it is not true unless stable page supports. So, If the data is changed under us, zram can make buffer overrun because second compression size could be bigger than one we got in previous trial and blindly, copy bigger size object to smaller buffer which is buffer overrun. The overrun breaks zsmalloc free object chaining so system goes crash like above. I think below is same problem. https://bugzilla.suse.com/show_bug.cgi?id=997574 Unfortunately, reuse_swap_page should be atomic so that we cannot wait on writeback in there so the approach in this patch is simply return false if we found it needs stable page. Although it increases memory footprint temporarily, it happens rarely and it should be reclaimed easily althoug it happened. Also, It would be better than waiting of IO completion, which is critial path for application latency. Fixes: da9556a2367c ("zram: user per-cpu compression streams") Link: http://lkml.kernel.org/r/20161120233015.GA14113@bbox Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx> Cc: linux-mm@xxxxxxxxx Cc: Darrick J. Wong <darrick.wong@xxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx --- include/linux/swap.h | 3 ++- mm/swapfile.c | 20 +++++++++++++++++++- 2 files changed, 21 insertions(+), 2 deletions(-) diff --git a/include/linux/swap.h b/include/linux/swap.h index 09f4be179ff3..7f47b7098b1b 100644 --- a/include/linux/swap.h +++ b/include/linux/swap.h @@ -150,8 +150,9 @@ enum { SWP_FILE = (1 << 7), /* set after swap_activate success */ SWP_AREA_DISCARD = (1 << 8), /* single-time swap area discards */ SWP_PAGE_DISCARD = (1 << 9), /* freed swap page-cluster discards */ + SWP_STABLE_WRITES = (1 << 10), /* no overwrite PG_writeback pages */ /* add others here before... */ - SWP_SCANNING = (1 << 10), /* refcount in scan_swap_map */ + SWP_SCANNING = (1 << 11), /* refcount in scan_swap_map */ }; #define SWAP_CLUSTER_MAX 32UL diff --git a/mm/swapfile.c b/mm/swapfile.c index f30438970cd1..d76b2a18f044 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -943,11 +943,25 @@ bool reuse_swap_page(struct page *page, int *total_mapcount) count = page_trans_huge_mapcount(page, total_mapcount); if (count <= 1 && PageSwapCache(page)) { count += page_swapcount(page); - if (count == 1 && !PageWriteback(page)) { + if (count != 1) + goto out; + if (!PageWriteback(page)) { delete_from_swap_cache(page); SetPageDirty(page); + } else { + swp_entry_t entry; + struct swap_info_struct *p; + + entry.val = page_private(page); + p = swap_info_get(entry); + if (p->flags & SWP_STABLE_WRITES) { + spin_unlock(&p->lock); + return false; + } + spin_unlock(&p->lock); } } +out: return count <= 1; } @@ -2449,6 +2463,10 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags) error = -ENOMEM; goto bad_swap; } + + if (bdi_cap_stable_pages_required(inode_to_bdi(inode))) + p->flags |= SWP_STABLE_WRITES; + if (p->bdev && blk_queue_nonrot(bdev_get_queue(p->bdev))) { int cpu; -- 2.7.4 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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