• Vyacheslav Dubeyko's avatar
    nilfs2: fix issue with race condition of competition between segments for dirty blocks · 7f42ec39
    Vyacheslav Dubeyko authored
    Many NILFS2 users were reported about strange file system corruption
    (for example):
    
       NILFS: bad btree node (blocknr=185027): level = 0, flags = 0x0, nchildren = 768
       NILFS error (device sda4): nilfs_bmap_last_key: broken bmap (inode number=11540)
    
    But such error messages are consequence of file system's issue that takes
    place more earlier.  Fortunately, Jerome Poulin <jeromepoulin@gmail.com>
    and Anton Eliasson <devel@antoneliasson.se> were reported about another
    issue not so recently.  These reports describe the issue with segctor
    thread's crash:
    
      BUG: unable to handle kernel paging request at 0000000000004c83
      IP: nilfs_end_page_io+0x12/0xd0 [nilfs2]
    
      Call Trace:
       nilfs_segctor_do_construct+0xf25/0x1b20 [nilfs2]
       nilfs_segctor_construct+0x17b/0x290 [nilfs2]
       nilfs_segctor_thread+0x122/0x3b0 [nilfs2]
       kthread+0xc0/0xd0
       ret_from_fork+0x7c/0xb0
    
    These two issues have one reason.  This reason can raise third issue
    too.  Third issue results in hanging of segctor thread with eating of
    100% CPU.
    
    REPRODUCING PATH:
    
    One of the possible way or the issue reproducing was described by
    Jermoe me Poulin <jeromepoulin@gmail.com>:
    
    1. init S to get to single user mode.
    2. sysrq+E to make sure only my shell is running
    3. start network-manager to get my wifi connection up
    4. login as root and launch "screen"
    5. cd /boot/log/nilfs which is a ext3 mount point and can log when NILFS dies.
    6. lscp | xz -9e > lscp.txt.xz
    7. mount my snapshot using mount -o cp=3360839,ro /dev/vgUbuntu/root /mnt/nilfs
    8. start a screen to dump /proc/kmsg to text file since rsyslog is killed
    9. start a screen and launch strace -f -o find-cat.log -t find
    /mnt/nilfs -type f -exec cat {} > /dev/null \;
    10. start a screen and launch strace -f -o apt-get.log -t apt-get update
    11. launch the last command again as it did not crash the first time
    12. apt-get crashes
    13. ps aux > ps-aux-crashed.log
    13. sysrq+W
    14. sysrq+E  wait for everything to terminate
    15. sysrq+SUSB
    
    Simplified way of the issue reproducing is starting kernel compilation
    task and "apt-get update" in parallel.
    
    REPRODUCIBILITY:
    
    The issue is reproduced not stable [60% - 80%].  It is very important to
    have proper environment for the issue reproducing.  The critical
    conditions for successful reproducing:
    
    (1) It should have big modified file by mmap() way.
    
    (2) This file should have the count of dirty blocks are greater that
        several segments in size (for example, two or three) from time to time
        during processing.
    
    (3) It should be intensive background activity of files modification
        in another thread.
    
    INVESTIGATION:
    
    First of all, it is possible to see that the reason of crash is not valid
    page address:
    
      NILFS [nilfs_segctor_complete_write]:2100 bh->b_count 0, bh->b_blocknr 13895680, bh->b_size 13897727, bh->b_page 0000000000001a82
      NILFS [nilfs_segctor_complete_write]:2101 segbuf->sb_segnum 6783
    
    Moreover, value of b_page (0x1a82) is 6786.  This value looks like segment
    number.  And b_blocknr with b_size values look like block numbers.  So,
    buffer_head's pointer points on not proper address value.
    
    Detailed investigation of the issue is discovered such picture:
    
      [-----------------------------SEGMENT 6783-------------------------------]
      NILFS [nilfs_segctor_do_construct]:2310 nilfs_segctor_begin_construction
      NILFS [nilfs_segctor_do_construct]:2321 nilfs_segctor_collect
      NILFS [nilfs_segctor_do_construct]:2336 nilfs_segctor_assign
      NILFS [nilfs_segctor_do_construct]:2367 nilfs_segctor_update_segusage
      NILFS [nilfs_segctor_do_construct]:2371 nilfs_segctor_prepare_write
      NILFS [nilfs_segctor_do_construct]:2376 nilfs_add_checksums_on_logs
      NILFS [nilfs_segctor_do_construct]:2381 nilfs_segctor_write
      NILFS [nilfs_segbuf_submit_bio]:464 bio->bi_sector 111149024, segbuf->sb_segnum 6783
    
      [-----------------------------SEGMENT 6784-------------------------------]
      NILFS [nilfs_segctor_do_construct]:2310 nilfs_segctor_begin_construction
      NILFS [nilfs_segctor_do_construct]:2321 nilfs_segctor_collect
      NILFS [nilfs_lookup_dirty_data_buffers]:782 bh->b_count 1, bh->b_page ffffea000709b000, page->index 0, i_ino 1033103, i_size 25165824
      NILFS [nilfs_lookup_dirty_data_buffers]:783 bh->b_assoc_buffers.next ffff8802174a6798, bh->b_assoc_buffers.prev ffff880221cffee8
      NILFS [nilfs_segctor_do_construct]:2336 nilfs_segctor_assign
      NILFS [nilfs_segctor_do_construct]:2367 nilfs_segctor_update_segusage
      NILFS [nilfs_segctor_do_construct]:2371 nilfs_segctor_prepare_write
      NILFS [nilfs_segctor_do_construct]:2376 nilfs_add_checksums_on_logs
      NILFS [nilfs_segctor_do_construct]:2381 nilfs_segctor_write
      NILFS [nilfs_segbuf_submit_bh]:575 bh->b_count 1, bh->b_page ffffea000709b000, page->index 0, i_ino 1033103, i_size 25165824
      NILFS [nilfs_segbuf_submit_bh]:576 segbuf->sb_segnum 6784
      NILFS [nilfs_segbuf_submit_bh]:577 bh->b_assoc_buffers.next ffff880218a0d5f8, bh->b_assoc_buffers.prev ffff880218bcdf50
      NILFS [nilfs_segbuf_submit_bio]:464 bio->bi_sector 111150080, segbuf->sb_segnum 6784, segbuf->sb_nbio 0
      [----------] ditto
      NILFS [nilfs_segbuf_submit_bio]:464 bio->bi_sector 111164416, segbuf->sb_segnum 6784, segbuf->sb_nbio 15
    
      [-----------------------------SEGMENT 6785-------------------------------]
      NILFS [nilfs_segctor_do_construct]:2310 nilfs_segctor_begin_construction
      NILFS [nilfs_segctor_do_construct]:2321 nilfs_segctor_collect
      NILFS [nilfs_lookup_dirty_data_buffers]:782 bh->b_count 2, bh->b_page ffffea000709b000, page->index 0, i_ino 1033103, i_size 25165824
      NILFS [nilfs_lookup_dirty_data_buffers]:783 bh->b_assoc_buffers.next ffff880219277e80, bh->b_assoc_buffers.prev ffff880221cffc88
      NILFS [nilfs_segctor_do_construct]:2367 nilfs_segctor_update_segusage
      NILFS [nilfs_segctor_do_construct]:2371 nilfs_segctor_prepare_write
      NILFS [nilfs_segctor_do_construct]:2376 nilfs_add_checksums_on_logs
      NILFS [nilfs_segctor_do_construct]:2381 nilfs_segctor_write
      NILFS [nilfs_segbuf_submit_bh]:575 bh->b_count 2, bh->b_page ffffea000709b000, page->index 0, i_ino 1033103, i_size 25165824
      NILFS [nilfs_segbuf_submit_bh]:576 segbuf->sb_segnum 6785
      NILFS [nilfs_segbuf_submit_bh]:577 bh->b_assoc_buffers.next ffff880218a0d5f8, bh->b_assoc_buffers.prev ffff880222cc7ee8
      NILFS [nilfs_segbuf_submit_bio]:464 bio->bi_sector 111165440, segbuf->sb_segnum 6785, segbuf->sb_nbio 0
      [----------] ditto
      NILFS [nilfs_segbuf_submit_bio]:464 bio->bi_sector 111177728, segbuf->sb_segnum 6785, segbuf->sb_nbio 12
    
      NILFS [nilfs_segctor_do_construct]:2399 nilfs_segctor_wait
      NILFS [nilfs_segbuf_wait]:676 segbuf->sb_segnum 6783
      NILFS [nilfs_segbuf_wait]:676 segbuf->sb_segnum 6784
      NILFS [nilfs_segbuf_wait]:676 segbuf->sb_segnum 6785
    
      NILFS [nilfs_segctor_complete_write]:2100 bh->b_count 0, bh->b_blocknr 13895680, bh->b_size 13897727, bh->b_page 0000000000001a82
    
      BUG: unable to handle kernel paging request at 0000000000001a82
      IP: [<ffffffffa024d0f2>] nilfs_end_page_io+0x12/0xd0 [nilfs2]
    
    Usually, for every segment we collect dirty files in list.  Then, dirty
    blocks are gathered for every dirty file, prepared for write and
    submitted by means of nilfs_segbuf_submit_bh() call.  Finally, it takes
    place complete write phase after calling nilfs_end_bio_write() on the
    block layer.  Buffers/pages are marked as not dirty on final phase and
    processed files removed from the list of dirty files.
    
    It is possible to see that we had three prepare_write and submit_bio
    phases before segbuf_wait and complete_write phase.  Moreover, segments
    compete between each other for dirty blocks because on every iteration
    of segments processing dirty buffer_heads are added in several lists of
    payload_buffers:
    
      [SEGMENT 6784]: bh->b_assoc_buffers.next ffff880218a0d5f8, bh->b_assoc_buffers.prev ffff880218bcdf50
      [SEGMENT 6785]: bh->b_assoc_buffers.next ffff880218a0d5f8, bh->b_assoc_buffers.prev ffff880222cc7ee8
    
    The next pointer is the same but prev pointer has changed.  It means
    that buffer_head has next pointer from one list but prev pointer from
    another.  Such modification can be made several times.  And, finally, it
    can be resulted in various issues: (1) segctor hanging, (2) segctor
    crashing, (3) file system metadata corruption.
    
    FIX:
    This patch adds:
    
    (1) setting of BH_Async_Write flag in nilfs_segctor_prepare_write()
        for every proccessed dirty block;
    
    (2) checking of BH_Async_Write flag in
        nilfs_lookup_dirty_data_buffers() and
        nilfs_lookup_dirty_node_buffers();
    
    (3) clearing of BH_Async_Write flag in nilfs_segctor_complete_write(),
        nilfs_abort_logs(), nilfs_forget_buffer(), nilfs_clear_dirty_page().
    Reported-by: 's avatarJerome Poulin <jeromepoulin@gmail.com>
    Reported-by: 's avatarAnton Eliasson <devel@antoneliasson.se>
    Cc: Paul Fertser <fercerpav@gmail.com>
    Cc: ARAI Shun-ichi <hermes@ceres.dti.ne.jp>
    Cc: Piotr Szymaniak <szarpaj@grubelek.pl>
    Cc: Juan Barry Manuel Canham <Linux@riotingpacifist.net>
    Cc: Zahid Chowdhury <zahid.chowdhury@starsolutions.com>
    Cc: Elmer Zhang <freeboy6716@gmail.com>
    Cc: Kenneth Langga <klangga@gmail.com>
    Signed-off-by: 's avatarVyacheslav Dubeyko <slava@dubeyko.com>
    Acked-by: 's avatarRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
    7f42ec39
Name
Last commit
Last update
..
Kconfig Loading commit data...
Makefile Loading commit data...
alloc.c Loading commit data...
alloc.h Loading commit data...
bmap.c Loading commit data...
bmap.h Loading commit data...
btnode.c Loading commit data...
btnode.h Loading commit data...
btree.c Loading commit data...
btree.h Loading commit data...
cpfile.c Loading commit data...
cpfile.h Loading commit data...
dat.c Loading commit data...
dat.h Loading commit data...
dir.c Loading commit data...
direct.c Loading commit data...
direct.h Loading commit data...
export.h Loading commit data...
file.c Loading commit data...
gcinode.c Loading commit data...
ifile.c Loading commit data...
ifile.h Loading commit data...
inode.c Loading commit data...
ioctl.c Loading commit data...
mdt.c Loading commit data...
mdt.h Loading commit data...
namei.c Loading commit data...
nilfs.h Loading commit data...
page.c Loading commit data...
page.h Loading commit data...
recovery.c Loading commit data...
segbuf.c Loading commit data...
segbuf.h Loading commit data...
segment.c Loading commit data...
segment.h Loading commit data...
sufile.c Loading commit data...
sufile.h Loading commit data...
super.c Loading commit data...
the_nilfs.c Loading commit data...
the_nilfs.h Loading commit data...