1. 24 Jan, 2022 1 commit
    • Filipe Manana's avatar
      btrfs: fix deadlock when reserving space during defrag · 0cb5950f
      Filipe Manana authored
      When defragging we can end up collecting a range for defrag that has
      already pages under delalloc (dirty), as long as the respective extent
      map for their range is not mapped to a hole, a prealloc extent or
      the extent map is from an old generation.
      
      Most of the time that is harmless from a functional perspective at
      least, however it can result in a deadlock:
      
      1) At defrag_collect_targets() we find an extent map that meets all
         requirements but there's delalloc for the range it covers, and we add
         its range to list of ranges to defrag;
      
      2) The defrag_collect_targets() function is called at defrag_one_range(),
         after it locked a range that overlaps the range of the extent map;
      
      3) At defrag_one_range(), while the range is still locked, we call
         defrag_one_locked_target() for the range associated to the extent
         map we collected at step 1);
      
      4) Then finally at defrag_one_locked_target() we do a call to
         btrfs_delalloc_reserve_space(), which will reserve data and metadata
         space. If the space reservations can not be satisfied right away, the
         flusher might be kicked in and start flushing delalloc and wait for
         the respective ordered extents to complete. If this happens we will
         deadlock, because both flushing delalloc and finishing an ordered
         extent, requires locking the range in the inode's io tree, which was
         already locked at defrag_collect_targets().
      
      So fix this by skipping extent maps for which there's already delalloc.
      
      Fixes: eb793cf8 ("btrfs: defrag: introduce helper to collect target file extents")
      CC: stable@vger.kernel.org # 5.16
      Reviewed-by: default avatarQu Wenruo <wqu@suse.com>
      Signed-off-by: default avatarFilipe Manana <fdmanana@suse.com>
      Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
      0cb5950f
  2. 19 Jan, 2022 4 commits
    • Qu Wenruo's avatar
      btrfs: defrag: properly update range->start for autodefrag · c080b414
      Qu Wenruo authored
      [BUG]
      After commit 7b508037 ("btrfs: defrag: use defrag_one_cluster() to
      implement btrfs_defrag_file()") autodefrag no longer properly re-defrag
      the file from previously finished location.
      
      [CAUSE]
      The recent refactoring of defrag only focuses on defrag ioctl subpage
      support, doesn't take autodefrag into consideration.
      
      There are two problems involved which prevents autodefrag to restart its
      scan:
      
      - No range.start update
        Previously when one defrag target is found, range->start will be
        updated to indicate where next search should start from.
      
        But now btrfs_defrag_file() doesn't update it anymore, making all
        autodefrag to rescan from file offset 0.
      
        This would also make autodefrag to mark the same range dirty again and
        again, causing extra IO.
      
      - No proper quick exit for defrag_one_cluster()
        Currently if we reached or exceed @max_sectors limit, we just exit
        defrag_one_cluster(), and let next defrag_one_cluster() call to do a
        quick exit.
        This makes @cur increase, thus no way to properly know which range is
        defragged and which range is skipped.
      
      [FIX]
      The fix involves two modifications:
      
      - Update range->start to next cluster start
        This is a little different from the old behavior.
        Previously range->start is updated to the next defrag target.
      
        But in the end, the behavior should still be pretty much the same,
        as now we skip to next defrag target inside btrfs_defrag_file().
      
        Thus if auto-defrag determines to re-scan, then we still do the skip,
        just at a different timing.
      
      - Make defrag_one_cluster() to return >0 to indicate a quick exit
        So that btrfs_defrag_file() can also do a quick exit, without
        increasing @cur to the range end, and re-use @cur to update
        @range->start.
      
      - Add comment for btrfs_defrag_file() to mention the range->start update
        Currently only autodefrag utilize this behavior, as defrag ioctl won't
        set @max_to_defrag parameter, thus unless interrupted it will always
        try to defrag the whole range.
      Reported-by: default avatarFilipe Manana <fdmanana@suse.com>
      Fixes: 7b508037 ("btrfs: defrag: use defrag_one_cluster() to implement btrfs_defrag_file()")
      Link: https://lore.kernel.org/linux-btrfs/0a269612-e43f-da22-c5bc-b34b1b56ebe8@mailbox.org/
      CC: stable@vger.kernel.org # 5.16
      Reviewed-by: default avatarFilipe Manana <fdmanana@suse.com>
      Signed-off-by: default avatarQu Wenruo <wqu@suse.com>
      Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
      c080b414
    • Qu Wenruo's avatar
      btrfs: defrag: fix wrong number of defragged sectors · 484167da
      Qu Wenruo authored
      [BUG]
      There are users using autodefrag mount option reporting obvious increase
      in IO:
      
      > If I compare the write average (in total, I don't have it per process)
      > when taking idle periods on the same machine:
      >     Linux 5.16:
      >         without autodefrag: ~ 10KiB/s
      >         with autodefrag: between 1 and 2MiB/s.
      >
      >     Linux 5.15:
      >         with autodefrag:~ 10KiB/s (around the same as without
      > autodefrag on 5.16)
      
      [CAUSE]
      When autodefrag mount option is enabled, btrfs_defrag_file() will be
      called with @max_sectors = BTRFS_DEFRAG_BATCH (1024) to limit how many
      sectors we can defrag in one try.
      
      And then use the number of sectors defragged to determine if we need to
      re-defrag.
      
      But commit b18c3ab2 ("btrfs: defrag: introduce helper to defrag one
      cluster") uses wrong unit to increase @sectors_defragged, which should
      be in unit of sector, not byte.
      
      This means, if we have defragged any sector, then @sectors_defragged
      will be >= sectorsize (normally 4096), which is larger than
      BTRFS_DEFRAG_BATCH.
      
      This makes the @max_sectors check in defrag_one_cluster() to underflow,
      rendering the whole @max_sectors check useless.
      
      Thus causing way more IO for autodefrag mount options, as now there is
      no limit on how many sectors can really be defragged.
      
      [FIX]
      Fix the problems by:
      
      - Use sector as unit when increasing @sectors_defragged
      
      - Include @sectors_defragged > @max_sectors case to break the loop
      
      - Add extra comment on the return value of btrfs_defrag_file()
      Reported-by: default avatarAnthony Ruhier <aruhier@mailbox.org>
      Fixes: b18c3ab2 ("btrfs: defrag: introduce helper to defrag one cluster")
      Link: https://lore.kernel.org/linux-btrfs/0a269612-e43f-da22-c5bc-b34b1b56ebe8@mailbox.org/
      CC: stable@vger.kernel.org # 5.16
      Reviewed-by: default avatarFilipe Manana <fdmanana@suse.com>
      Signed-off-by: default avatarQu Wenruo <wqu@suse.com>
      Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
      484167da
    • Filipe Manana's avatar
      btrfs: allow defrag to be interruptible · b767c2fc
      Filipe Manana authored
      During defrag, at btrfs_defrag_file(), we have this loop that iterates
      over a file range in steps no larger than 256K subranges. If the range
      is too long, there's no way to interrupt it. So make the loop check in
      each iteration if there's signal pending, and if there is, break and
      return -AGAIN to userspace.
      
      Before kernel 5.16, we used to allow defrag to be cancelled through a
      signal, but that was lost with commit 7b508037 ("btrfs: defrag:
      use defrag_one_cluster() to implement btrfs_defrag_file()").
      
      This change adds back the possibility to cancel a defrag with a signal
      and keeps the same semantics, returning -EAGAIN to user space (and not
      the usually more expected -EINTR).
      
      This is also motivated by a recent bug on 5.16 where defragging a 1 byte
      file resulted in iterating from file range 0 to (u64)-1, as hitting the
      bug triggered a too long loop, basically requiring one to reboot the
      machine, as it was not possible to cancel defrag.
      
      Fixes: 7b508037 ("btrfs: defrag: use defrag_one_cluster() to implement btrfs_defrag_file()")
      CC: stable@vger.kernel.org # 5.16
      Reviewed-by: default avatarQu Wenruo <wqu@suse.com>
      Signed-off-by: default avatarFilipe Manana <fdmanana@suse.com>
      Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
      b767c2fc
    • Filipe Manana's avatar
      btrfs: fix too long loop when defragging a 1 byte file · 6b34cd8e
      Filipe Manana authored
      When attempting to defrag a file with a single byte, we can end up in a
      too long loop, which is nearly infinite because at btrfs_defrag_file()
      we end up with the variable last_byte assigned with a value of
      18446744073709551615 (which is (u64)-1). The problem comes from the fact
      we end up doing:
      
          last_byte = round_up(last_byte, fs_info->sectorsize) - 1;
      
      So if last_byte was assigned 0, which is i_size - 1, we underflow and
      end up with the value 18446744073709551615.
      
      This is trivial to reproduce and the following script triggers it:
      
        $ cat test.sh
        #!/bin/bash
      
        DEV=/dev/sdj
        MNT=/mnt/sdj
      
        mkfs.btrfs -f $DEV
        mount $DEV $MNT
      
        echo -n "X" > $MNT/foobar
      
        btrfs filesystem defragment $MNT/foobar
      
        umount $MNT
      
      So fix this by not decrementing last_byte by 1 before doing the sector
      size round up. Also, to make it easier to follow, make the round up right
      after computing last_byte.
      Reported-by: default avatarAnthony Ruhier <aruhier@mailbox.org>
      Fixes: 7b508037 ("btrfs: defrag: use defrag_one_cluster() to implement btrfs_defrag_file()")
      Link: https://lore.kernel.org/linux-btrfs/0a269612-e43f-da22-c5bc-b34b1b56ebe8@mailbox.org/
      CC: stable@vger.kernel.org # 5.16
      Reviewed-by: default avatarQu Wenruo <wqu@suse.com>
      Signed-off-by: default avatarFilipe Manana <fdmanana@suse.com>
      Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
      6b34cd8e
  3. 07 Jan, 2022 35 commits