btrfs: allow btrfs_truncate_block() to fallback to nocow for data space reservation

[BUG]
When the data space is exhausted, even if the inode has NOCOW attribute,
we will still refuse to truncate unaligned range due to ENOSPC.

The following script can reproduce it pretty easily:
  #!/bin/bash

  dev=/dev/test/test
  mnt=/mnt/btrfs

  umount $dev &> /dev/null
  umount $mnt &> /dev/null

  mkfs.btrfs -f $dev -b 1G
  mount -o nospace_cache $dev $mnt
  touch $mnt/foobar
  chattr +C $mnt/foobar

  xfs_io -f -c "pwrite -b 4k 0 4k" $mnt/foobar > /dev/null
  xfs_io -f -c "pwrite -b 4k 0 1G" $mnt/padding &> /dev/null
  sync

  xfs_io -c "fpunch 0 2k" $mnt/foobar
  umount $mnt

Currently this will fail at the fpunch part.

[CAUSE]
Because btrfs_truncate_block() always reserves space without checking
the NOCOW attribute.

Since the writeback path follows NOCOW bit, we only need to bother the
space reservation code in btrfs_truncate_block().

[FIX]
Make btrfs_truncate_block() follow btrfs_buffered_write() to try to
reserve data space first, and fall back to NOCOW check only when we
don't have enough space.

Such always-try-reserve is an optimization introduced in
btrfs_buffered_write(), to avoid expensive btrfs_check_can_nocow() call.

This patch will export check_can_nocow() as btrfs_check_can_nocow(), and
use it in btrfs_truncate_block() to fix the problem.

Reported-by: Martin Doucha <martin.doucha@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 033e411dd811..75dfb9b4d9e3 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -3033,6 +3033,8 @@ int btrfs_dirty_pages(struct inode *inode, struct page **pages,
 		      size_t num_pages, loff_t pos, size_t write_bytes,
 		      struct extent_state **cached);
 int btrfs_fdatawrite_range(struct inode *inode, loff_t start, loff_t end);
+int btrfs_check_can_nocow(struct btrfs_inode *inode, loff_t pos,
+			  size_t *write_bytes, bool nowait);
 
 /* tree-defrag.c */
 int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,