From: Ross Zwisler Subject: Re: [PATCH 0/6] ext4: Punch hole and DAX fixes Date: Wed, 14 Oct 2015 15:07:07 -0600 Message-ID: <20151014210707.GA18639@linux.intel.com> References: <1444822227-29984-1-git-send-email-jack@suse.com> <20151014180604.GA31803@linux.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kara , linux-ext4@vger.kernel.org, Dan Williams , Matthew Wilcox , Dave Chinner To: Ross Zwisler Return-path: Received: from mga11.intel.com ([192.55.52.93]:34320 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754513AbbJNVHI (ORCPT ); Wed, 14 Oct 2015 17:07:08 -0400 Content-Disposition: inline In-Reply-To: <20151014180604.GA31803@linux.intel.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed, Oct 14, 2015 at 12:06:04PM -0600, Ross Zwisler wrote: > On Wed, Oct 14, 2015 at 01:30:21PM +0200, Jan Kara wrote: > > Hello, > > > > This series fixes a long standing problem of racing punch hole and page fault > > resulting in possible filesystem corruption or stale data exposure. We fix the > > problem by using a new inode-private rw_semaphore i_mmap_sem to synchronize > > page faults with truncate and punch hole operations. > > > > When having this exclusion, the only remaining problem with DAX implementation > > are races between two page faults zeroing out same block concurrently (where > > the data written after the first fault finishes are possibly overwritten by > > the second fault still doing zeroing). That is dealt with by patches 5 and 6 > > in this series where we implement block zeroing directly in ext4_map_blocks(). > > > > The patches survived xfstests run both in dax and non-dax mode. > > > > Honza > > I threw these patches in to my own test setup and was able to hit the > following with generic/198: > > [ 57.453855] run fstests generic/198 at 2015-10-14 11:41:21 > [ 57.585886] ------------[ cut here ]------------ > [ 57.585890] kernel BUG at fs/ext4/inode.c:3057! > [ 57.585892] invalid opcode: 0000 [#1] SMP > [ 57.585894] Modules linked in: nd_pmem nd_btt nd_e820 libnvdimm > [ 57.585898] CPU: 0 PID: 3099 Comm: aiodio_sparse2 Not tainted 4.3.0-rc4_ext4_locking+ #1 > [ 57.585899] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140709_153950- 04/01/2014 > [ 57.585900] task: ffff88040ea3b400 ti: ffff8800ba5e8000 task.ti: ffff8800ba5e8000 > [ 57.585901] RIP: 0010:[] [] ext4_dax_mmap_get_block+0x1f9/0x250 > [ 57.585906] RSP: 0000:ffff8800ba5ebc48 EFLAGS: 00010216 > [ 57.585907] RAX: 0000000000000200 RBX: ffff8800ba5ebcf8 RCX: 000000000000000c > [ 57.585908] RDX: ffff8800ba5ebcf8 RSI: 000000000000004b RDI: ffff8801fbdb13c0 > [ 57.585909] RBP: ffff8800ba5ebca0 R08: ffffffff812f7980 R09: ffff8801fbdb15e0 > [ 57.585910] R10: 00007f7921c00000 R11: 000000000000000c R12: ffff8801fbdb13c0 > [ 57.585911] R13: 0000000000000001 R14: ffff8800b153c870 R15: 0000000000000000 > [ 57.585912] FS: 00007f7928999700(0000) GS:ffff880411000000(0000) knlGS:0000000000000000 > [ 57.585913] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [ 57.585914] CR2: 00007f7921c00000 CR3: 00000000bb1b5000 CR4: 00000000000406f0 > [ 57.585917] Stack: > [ 57.585918] ffff8800ba351000 ffff8800bafca028 0000000000000000 000002000000004b > [ 57.585920] ffff8800ba5ebd08 000000004edcd27a ffff8800baf47f18 ffff8801fbdb13c0 > [ 57.585922] 00007f7921c00000 ffff8800b153c870 0000000000000000 ffff8800ba5ebd90 > [ 57.585924] Call Trace: > [ 57.585928] [] __dax_pmd_fault+0x15c/0x590 > [ 57.585931] [] ? kmem_cache_alloc+0xd6/0x210 > [ 57.585934] [] ext4_dax_pmd_fault+0xbb/0x140 > [ 57.585936] [] handle_mm_fault+0x2cb/0x1960 > [ 57.585938] [] ? handle_mm_fault+0x5a/0x1960 > [ 57.585941] [] __do_page_fault+0x191/0x3f0 > [ 57.585943] [] trace_do_page_fault+0x4f/0x120 > [ 57.585944] [] do_async_page_fault+0x1a/0xa0 > [ 57.585947] [] async_page_fault+0x28/0x30 > [ 57.585948] Code: e8 3d ef ff ff 85 c0 41 89 c5 4c 89 fa be 18 0c 00 00 48 c7 c7 50 0a c3 81 78 31 e8 82 17 03 00 85 c0 44 0f 48 f0 e9 98 fe ff ff <0f> 0b be 1d 0c 00 00 48 c7 c7 f8 6e f2 81 e8 74 b6 da ff c6 05 > [ 57.585969] RIP [] ext4_dax_mmap_get_block+0x1f9/0x250 > [ 57.585971] RSP > [ 57.585973] ---[ end trace 356a2226c61d7104 ]--- > > I'll spend some time today trying to figure out what's going on, but thought > I'd let you know early in case you were already debugging this. > > My test setup is a pair of 4 GiB PMEM partitions in a KVM guest, one for the > TEST_DEV and the other for the SCRATCH_DEV. I'm setting "-o dax" with > MOUNT_OPTIONS: > > # ./check generic/198 > FSTYP -- ext4 > PLATFORM -- Linux/x86_64 alara 4.3.0-rc4_ext4_locking+ > MKFS_OPTIONS -- /dev/pmem0p2 > MOUNT_OPTIONS -- -o dax -o context=system_u:object_r:nfs_t:s0 /dev/pmem0p2 /mnt/xfstests_scratch > > generic/198 0s ... [failed, exit status 139] - output mismatch (see /root/xfstests/results//generic/198.out.bad) > --- tests/generic/198.out 2015-10-02 10:19:36.804795880 -0600 > +++ /root/xfstests/results//generic/198.out.bad 2015-10-14 11:41:21.234972535 -0600 > @@ -1,2 +1,3 @@ > QA output created by 198 > Silence is golden. > +./tests/generic/198: line 55: 3099 Segmentation fault $AIO_TEST "$TEST_DIR/aiodio_sparse" > ... > (Run 'diff -u tests/generic/198.out /root/xfstests/results//generic/198.out.bad' to see the entire diff) It looks like the issue is that we're running through the DAX PMD fault path, so we are trying to map 2 MiB at once. ext4_dax_mmap_get_block() enforces an assumption that it will only be called for a single block at a time with BUG_ON(map.m_len != 1); When running through the PMD path, though, map.m_len is 512. I'm guessing that you didn't hit this in your testing because things weren't set up in such a way that you received PMD faults. To be able to get this to work in my setup I reserve PMEM starting at an aligned address (I have it at a 2GiB alignment, something smaller might be sufficient) and I set up my partitions to be on 1GiB alignments as well. There may be more precise ways to get this to to work. :) For ease of copy and paste, my mmap param for PMEM is "memmap=8G!8G" (I have 16 GiB of RAM in my VM), and here is the bash function I use to set up my test environment: function xfstests_init { umount /dev/pmem* 2>/dev/null parted -s $xfstests_dev mktable msdos parted -s -a optimal $xfstests_dev mkpart Primary 1 4GiB parted -s -a optimal $xfstests_dev mkpart Primary 4GiB 7GiB if [[ $test_fs == "xfs" ]]; then mkfs.xfs -f $TEST_DEV mkfs.xfs -f $SCRATCH_DEV elif [[ $test_fs == "ext4" ]]; then mkfs.ext4 -F $TEST_DEV mkfs.ext4 -F $SCRATCH_DEV elif [[ $test_fs == "ext2" ]]; then mkfs.ext2 -F $TEST_DEV mkfs.ext2 -F $SCRATCH_DEV else echo "Invalid test fs '$test_fs'" fi }