[v3,1/4] Btrfs: fix missing file extent item for hole after ranged fsync

Commit Message

Filipe Manana March 9, 2020, 12:41 p.m. UTC

From: Filipe Manana <[e-mail]>

When doing a fast fsync for a range that starts at an offset greater than
zero, we can end up with a log that when replayed causes the respective
inode miss a file extent item representing a hole if we are not using the
NO_HOLES feature. This is because for fast fsyncs we don't log any extents
that cover a range different from the one requested in the fsync.

Example scenario to trigger it:

  $ mkfs.btrfs -O ^no-holes -f /dev/sdd
  $ mount /dev/sdd /mnt

  # Create a file with a single 256K and fsync it to clear to full sync
  # bit in the inode - we want the msync below to trigger a fast fsync.
  $ xfs_io -f -c "pwrite -S 0xab 0 256K" -c "fsync" /mnt/foo

  # Force a transaction commit and wipe out the log tree.
  $ sync

  # Dirty 768K of data, increasing the file size to 1Mb, and flush only
  # the range from 256K to 512K without updating the log tree
  # (sync_file_range() does not trigger fsync, it only starts writeback
  # and waits for it to finish).

  $ xfs_io -c "pwrite -S 0xcd 256K 768K" /mnt/foo
  $ xfs_io -c "sync_range -abw 256K 256K" /mnt/foo

  # Now dirty the range from 768K to 1M again and sync that range.
  $ xfs_io -c "mmap -w 768K 256K"        \
           -c "mwrite -S 0xef 768K 256K" \
           -c "msync -s 768K 256K"       \
           -c "munmap"                   \
           /mnt/foo

  <power fail>

  # Mount to replay the log.
  $ mount /dev/sdd /mnt
  $ umount /mnt

  $ btrfs check /dev/sdd
  Opening filesystem to check...
  Checking filesystem on /dev/sdd
  UUID: 482fb574-b288-478e-a190-a9c44a78fca6
  [1/7] checking root items
  [2/7] checking extents
  [3/7] checking free space cache
  [4/7] checking fs roots
  root 5 inode 257 errors 100, file extent discount
  Found file extent holes:
       start: 262144, len: 524288
  ERROR: errors found in fs roots
  found 720896 bytes used, error(s) found
  total csum bytes: 512
  total tree bytes: 131072
  total fs tree bytes: 32768
  total extent tree bytes: 16384
  btree space waste bytes: 123514
  file data blocks allocated: 589824
    referenced 589824

Fix this issue by setting the range to full (0 to LLONG_MAX) when the
NO_HOLES feature is not enabled. This results in extra work being done
but it gives the guarantee we don't end up with missing holes after
replaying the log.

Signed-off-by: Filipe Manana <[e-mail]>
---
 fs/btrfs/file.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

Patch

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 31c72371a164..4a536387e992 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -2072,6 +2072,16 @@  int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
 	btrfs_init_log_ctx(&ctx, inode);
 
 	/*
+	 * Set the range to full if the NO_HOLES feature is not enabled.
+	 * This is to avoid missing file extent items representing holes after
+	 * replaying the log.
+	 */
+	if (!btrfs_fs_incompat(fs_info, NO_HOLES)) {
+		start = 0;
+		end = LLONG_MAX;
+	}
+
+	/*
 	 * We write the dirty pages in the range and wait until they complete
 	 * out of the ->i_mutex. If so, we can flush the dirty pages by
 	 * multi-task, and make the performance up.  See