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Subject[PATCH 5.4 097/135] Btrfs: fix race between shrinking truncate and fiemap
From: Filipe Manana <>

commit 28553fa992cb28be6a65566681aac6cafabb4f2d upstream.

When there is a fiemap executing in parallel with a shrinking truncate
we can end up in a situation where we have extent maps for which we no
longer have corresponding file extent items. This is generally harmless
and at the moment the only consequences are missing file extent items
representing holes after we expand the file size again after the
truncate operation removed the prealloc extent items, and stale
information for future fiemap calls (reporting extents that no longer
exist or may have been reallocated to other files for example).

Consider the following example:

1) Our inode has a size of 128KiB, one 128KiB extent at file offset 0
and a 1MiB prealloc extent at file offset 128KiB;

2) Task A starts doing a shrinking truncate of our inode to reduce it to
a size of 64KiB. Before it searches the subvolume tree for file
extent items to delete, it drops all the extent maps in the range
from 64KiB to (u64)-1 by calling btrfs_drop_extent_cache();

3) Task B starts doing a fiemap against our inode. When looking up for
the inode's extent maps in the range from 128KiB to (u64)-1, it
doesn't find any in the inode's extent map tree, since they were
removed by task A. Because it didn't find any in the extent map
tree, it scans the inode's subvolume tree for file extent items, and
it finds the 1MiB prealloc extent at file offset 128KiB, then it
creates an extent map based on that file extent item and adds it to
inode's extent map tree (this ends up being done by
btrfs_get_extent() <- btrfs_get_extent_fiemap() <-

4) Task A then drops the prealloc extent at file offset 128KiB and
shrinks the 128KiB extent file offset 0 to a length of 64KiB. The
truncation operation finishes and we end up with an extent map
representing a 1MiB prealloc extent at file offset 128KiB, despite we
don't have any more that extent;

After this the two types of problems we have are:

1) Future calls to fiemap always report that a 1MiB prealloc extent
exists at file offset 128KiB. This is stale information, no longer

2) If the size of the file is increased, by a truncate operation that
increases the file size or by a write into a file offset > 64KiB for
example, we end up not inserting file extent items to represent holes
for any range between 128KiB and 128KiB + 1MiB, since the hole
expansion function, btrfs_cont_expand() will skip hole insertion for
any range for which an extent map exists that represents a prealloc
extent. This causes fsck to complain about missing file extent items
when not using the NO_HOLES feature.

The second issue could be often triggered by test case generic/561 from
fstests, which runs fsstress and duperemove in parallel, and duperemove
does frequent fiemap calls.

Essentially the problems happens because fiemap does not acquire the
inode's lock while truncate does, and fiemap locks the file range in the
inode's iotree while truncate does not. So fix the issue by making
btrfs_truncate_inode_items() lock the file range from the new file size
to (u64)-1, so that it serializes with fiemap.

CC: # 4.4+
Reviewed-by: Josef Bacik <>
Signed-off-by: Filipe Manana <>
Reviewed-by: David Sterba <>
Signed-off-by: David Sterba <>
Signed-off-by: Greg Kroah-Hartman <>

fs/btrfs/inode.c | 8 ++++++++
1 file changed, 8 insertions(+)

--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -4734,6 +4734,8 @@ int btrfs_truncate_inode_items(struct bt
u64 bytes_deleted = 0;
bool be_nice = false;
bool should_throttle = false;
+ const u64 lock_start = ALIGN_DOWN(new_size, fs_info->sectorsize);
+ struct extent_state *cached_state = NULL;

BUG_ON(new_size > 0 && min_type != BTRFS_EXTENT_DATA_KEY);

@@ -4750,6 +4752,9 @@ int btrfs_truncate_inode_items(struct bt
return -ENOMEM;
path->reada = READA_BACK;

+ lock_extent_bits(&BTRFS_I(inode)->io_tree, lock_start, (u64)-1,
+ &cached_state);
* We want to drop from the next block forward in case this new size is
* not block aligned since we will be keeping the last block of the
@@ -5016,6 +5021,9 @@ out:
btrfs_ordered_update_i_size(inode, last_size, NULL);

+ unlock_extent_cached(&BTRFS_I(inode)->io_tree, lock_start, (u64)-1,
+ &cached_state);
return ret;

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