btrfs: update btrfs_space_info's bytes_may_use timely

This patch can fix some false ENOSPC errors, below test script can
reproduce one false ENOSPC error:
	#!/bin/bash
	dd if=/dev/zero of=fs.img bs=$((1024*1024)) count=128
	dev=$(losetup --show -f fs.img)
	mkfs.btrfs -f -M $dev
	mkdir /tmp/mntpoint
	mount $dev /tmp/mntpoint
	cd /tmp/mntpoint
	xfs_io -f -c "falloc 0 $((64*1024*1024))" testfile

Above script will fail for ENOSPC reason, but indeed fs still has free
space to satisfy this request. Please see call graph:
btrfs_fallocate()
|-> btrfs_alloc_data_chunk_ondemand()
|   bytes_may_use += 64M
|-> btrfs_prealloc_file_range()
    |-> btrfs_reserve_extent()
        |-> btrfs_add_reserved_bytes()
        |   alloc_type is RESERVE_ALLOC_NO_ACCOUNT, so it does not
        |   change bytes_may_use, and bytes_reserved += 64M. Now
        |   bytes_may_use + bytes_reserved == 128M, which is greater
        |   than btrfs_space_info's total_bytes, false enospc occurs.
        |   Note, the bytes_may_use decrease operation will be done in
        |   end of btrfs_fallocate(), which is too late.

Here is another simple case for buffered write:
                    CPU 1              |              CPU 2
                                       |
|-> cow_file_range()                   |-> __btrfs_buffered_write()
    |-> btrfs_reserve_extent()         |   |
    |                                  |   |
    |                                  |   |
    |    .....                         |   |-> btrfs_check_data_free_space()
    |                                  |
    |                                  |
    |-> extent_clear_unlock_delalloc() |

In CPU 1, btrfs_reserve_extent()->find_free_extent()->
btrfs_add_reserved_bytes() do not decrease bytes_may_use, the decrease
operation will be delayed to be done in extent_clear_unlock_delalloc().
Assume in this case, btrfs_reserve_extent() reserved 128MB data, CPU2's
btrfs_check_data_free_space() tries to reserve 100MB data space.
If
	100MB > data_sinfo->total_bytes - data_sinfo->bytes_used -
		data_sinfo->bytes_reserved - data_sinfo->bytes_pinned -
		data_sinfo->bytes_readonly - data_sinfo->bytes_may_use
btrfs_check_data_free_space() will try to allcate new data chunk or call
btrfs_start_delalloc_roots(), or commit current transaction in order to
reserve some free space, obviously a lot of work. But indeed it's not
necessary as long as decreasing bytes_may_use timely, we still have
free space, decreasing 128M from bytes_may_use.

To fix this issue, this patch chooses to update bytes_may_use for both
data and metadata in btrfs_add_reserved_bytes(). For compress path, real
extent length may not be equal to file content length, so introduce a
ram_bytes argument for btrfs_reserve_extent(), find_free_extent() and
btrfs_add_reserved_bytes(), it's becasue bytes_may_use is increased by
file content length. Then compress path can update bytes_may_use
correctly. Also now we can discard RESERVE_ALLOC_NO_ACCOUNT, RESERVE_ALLOC
and RESERVE_FREE.

As we know, usually EXTENT_DO_ACCOUNTING is used for error path. In
run_delalloc_nocow(), for inode marked as NODATACOW or extent marked as
PREALLOC, we also need to update bytes_may_use, but can not pass
EXTENT_DO_ACCOUNTING, because it also clears metadata reservation, so
here we introduce EXTENT_CLEAR_DATA_RESV flag to indicate btrfs_clear_bit_hook()
to update btrfs_space_info's bytes_may_use.

Meanwhile __btrfs_prealloc_file_range() will call
btrfs_free_reserved_data_space() internally for both sucessful and failed
path, btrfs_prealloc_file_range()'s callers does not need to call
btrfs_free_reserved_data_space() any more.

Signed-off-by: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 15 insertions, 11 deletions

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 5842423f8f47..3391f2adf0c8 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -2675,6 +2675,7 @@ static long btrfs_fallocate(struct file *file, int mode,
 
 	alloc_start = round_down(offset, blocksize);
 	alloc_end = round_up(offset + len, blocksize);
+	cur_offset = alloc_start;
 
 	/* Make sure we aren't being give some crap mode */
 	if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE))
@@ -2767,7 +2768,6 @@ static long btrfs_fallocate(struct file *file, int mode,
 
 	/* First, check if we exceed the qgroup limit */
 	INIT_LIST_HEAD(&reserve_list);
-	cur_offset = alloc_start;
 	while (1) {
 		em = btrfs_get_extent(inode, NULL, 0, cur_offset,
 				      alloc_end - cur_offset, 0);
@@ -2794,6 +2794,14 @@ static long btrfs_fallocate(struct file *file, int mode,
 					last_byte - cur_offset);
 			if (ret < 0)
 				break;
+		} else {
+			/*
+			 * Do not need to reserve unwritten extent for this
+			 * range, free reserved data space first, otherwise
+			 * it'll result in false ENOSPC error.
+			 */
+			btrfs_free_reserved_data_space(inode, cur_offset,
+				last_byte - cur_offset);
 		}
 		free_extent_map(em);
 		cur_offset = last_byte;
@@ -2811,6 +2819,9 @@ static long btrfs_fallocate(struct file *file, int mode,
 					range->start,
 					range->len, 1 << inode->i_blkbits,
 					offset + len, &alloc_hint);
+		else
+			btrfs_free_reserved_data_space(inode, range->start,
+						       range->len);
 		list_del(&range->list);
 		kfree(range);
 	}
@@ -2845,18 +2856,11 @@ out_unlock:
 	unlock_extent_cached(&BTRFS_I(inode)->io_tree, alloc_start, locked_end,
 			     &cached_state, GFP_KERNEL);
 out:
-	/*
-	 * As we waited the extent range, the data_rsv_map must be empty
-	 * in the range, as written data range will be released from it.
-	 * And for prealloacted extent, it will also be released when
-	 * its metadata is written.
-	 * So this is completely used as cleanup.
-	 */
-	btrfs_qgroup_free_data(inode, alloc_start, alloc_end - alloc_start);
 	inode_unlock(inode);
 	/* Let go of our reservation. */
-	btrfs_free_reserved_data_space(inode, alloc_start,
-				       alloc_end - alloc_start);
+	if (ret != 0)
+		btrfs_free_reserved_data_space(inode, alloc_start,
+				       alloc_end - cur_offset);
 	return ret;
 }