xfs: Fix false ENOSPC when performing direct write on a delalloc extent in cow fork

On a higly fragmented filesystem a Direct IO write can fail with -ENOSPC error
even though the filesystem has sufficient number of free blocks.

This occurs if the file offset range on which the write operation is being
performed has a delalloc extent in the cow fork and this delalloc extent
begins much before the Direct IO range.

In such a scenario, xfs_reflink_allocate_cow() invokes xfs_bmapi_write() to
allocate the blocks mapped by the delalloc extent. The extent thus allocated
may not cover the beginning of file offset range on which the Direct IO write
was issued. Hence xfs_reflink_allocate_cow() ends up returning -ENOSPC.

The following script reliably recreates the bug described above.

  #!/usr/bin/bash

  device=/dev/loop0
  shortdev=$(basename $device)

  mntpnt=/mnt/
  file1=${mntpnt}/file1
  file2=${mntpnt}/file2
  fragmentedfile=${mntpnt}/fragmentedfile
  punchprog=/root/repos/xfstests-dev/src/punch-alternating

  errortag=/sys/fs/xfs/${shortdev}/errortag/bmap_alloc_minlen_extent

  umount $device > /dev/null 2>&1

  echo "Create FS"
  mkfs.xfs -f -m reflink=1 $device > /dev/null 2>&1
  if [[ $? != 0 ]]; then
  	echo "mkfs failed."
  	exit 1
  fi

  echo "Mount FS"
  mount $device $mntpnt > /dev/null 2>&1
  if [[ $? != 0 ]]; then
  	echo "mount failed."
  	exit 1
  fi

  echo "Create source file"
  xfs_io -f -c "pwrite 0 32M" $file1 > /dev/null 2>&1

  sync

  echo "Create Reflinked file"
  xfs_io -f -c "reflink $file1" $file2 &>/dev/null

  echo "Set cowextsize"
  xfs_io -c "cowextsize 16M" $file1 > /dev/null 2>&1

  echo "Fragment FS"
  xfs_io -f -c "pwrite 0 64M" $fragmentedfile > /dev/null 2>&1
  sync
  $punchprog $fragmentedfile

  echo "Allocate block sized extent from now onwards"
  echo -n 1 > $errortag

  echo "Create 16MiB delalloc extent in CoW fork"
  xfs_io -c "pwrite 0 4k" $file1 > /dev/null 2>&1

  sync

  echo "Direct I/O write at offset 12k"
  xfs_io -d -c "pwrite 12k 8k" $file1

This commit fixes the bug by invoking xfs_bmapi_write() in a loop until disk
blocks are allocated for atleast the starting file offset of the Direct IO
write range.

Fixes: 3c68d44a ("xfs: allocate direct I/O COW blocks in iomap_begin")
Reported-and-Root-caused-by: Wengang Wang <wen.gang.wang@oracle.com>
Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
[djwong: slight editing to make the locking less grody, and fix some style things]
Signed-off-by: Darrick J. Wong <djwong@kernel.org>