On Thu, Jun 25, 2015 at 04:17:46AM +0100, fdmanana@xxxxxxxxxx wrote:
> From: Filipe Manana <fdmanana@xxxxxxxx>
>
> When we have the no_holes feature enabled, if a we truncate a file to a
> smaller size, truncate it again but to a size greater than or equals to
> its original size and fsync it, the log tree will not have any information
> about the hole covering the range [truncate_1_offset, new_file_size[.
> Which means if the fsync log is replayed, the file will remain with the
> state it had before both truncate operations.
Does the fs/subvol tree get updated to the right information at this
time?
Thanks,
-liubo
>
> Without the no_holes feature this does not happen, since when the inode
> is logged (full sync flag is set) it will find in the fs/subvol tree a
> leaf with a generation matching the current transaction id that has an
> explicit extent item representing the hole.
>
> Fix this by adding an explicit extent item representing a hole between
> the last extent and the inode's i_size if we are doing a full sync.
>
> The issue is easy to reproduce with the following test case for fstests:
>
> . ./common/rc
> . ./common/filter
> . ./common/dmflakey
>
> _need_to_be_root
> _supported_fs generic
> _supported_os Linux
> _require_scratch
> _require_dm_flakey
>
> # This test was motivated by an issue found in btrfs when the btrfs
> # no-holes feature is enabled (introduced in kernel 3.14). So enable
> # the feature if the fs being tested is btrfs.
> if [ $FSTYP == "btrfs" ]; then
> _require_btrfs_fs_feature "no_holes"
> _require_btrfs_mkfs_feature "no-holes"
> MKFS_OPTIONS="$MKFS_OPTIONS -O no-holes"
> fi
>
> rm -f $seqres.full
>
> _scratch_mkfs >>$seqres.full 2>&1
> _init_flakey
> _mount_flakey
>
> # Create our test files and make sure everything is durably persisted.
> $XFS_IO_PROG -f -c "pwrite -S 0xaa 0 64K" \
> -c "pwrite -S 0xbb 64K 61K" \
> $SCRATCH_MNT/foo | _filter_xfs_io
> $XFS_IO_PROG -f -c "pwrite -S 0xee 0 64K" \
> -c "pwrite -S 0xff 64K 61K" \
> $SCRATCH_MNT/bar | _filter_xfs_io
> sync
>
> # Now truncate our file foo to a smaller size (64Kb) and then truncate
> # it to the size it had before the shrinking truncate (125Kb). Then
> # fsync our file. If a power failure happens after the fsync, we expect
> # our file to have a size of 125Kb, with the first 64Kb of data having
> # the value 0xaa and the second 61Kb of data having the value 0x00.
> $XFS_IO_PROG -c "truncate 64K" \
> -c "truncate 125K" \
> -c "fsync" \
> $SCRATCH_MNT/foo
>
> # Do something similar to our file bar, but the first truncation sets
> # the file size to 0 and the second truncation expands the size to the
> # double of what it was initially.
> $XFS_IO_PROG -c "truncate 0" \
> -c "truncate 253K" \
> -c "fsync" \
> $SCRATCH_MNT/bar
>
> _load_flakey_table $FLAKEY_DROP_WRITES
> _unmount_flakey
>
> # Allow writes again, mount to trigger log replay and validate file
> # contents.
> _load_flakey_table $FLAKEY_ALLOW_WRITES
> _mount_flakey
>
> # We expect foo to have a size of 125Kb, the first 64Kb of data all
> # having the value 0xaa and the remaining 61Kb to be a hole (all bytes
> # with value 0x00).
> echo "File foo content after log replay:"
> od -t x1 $SCRATCH_MNT/foo
>
> # We expect bar to have a size of 253Kb and no extents (any byte read
> # from bar has the value 0x00).
> echo "File bar content after log replay:"
> od -t x1 $SCRATCH_MNT/bar
>
> status=0
> exit
>
> The expected file contents in the golden output are:
>
> File foo content after log replay:
> 0000000 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
> *
> 0200000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> *
> 0372000
> File bar content after log replay:
> 0000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> *
> 0772000
>
> Without this fix, their contents are:
>
> File foo content after log replay:
> 0000000 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
> *
> 0200000 bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb
> *
> 0372000
> File bar content after log replay:
> 0000000 ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee
> *
> 0200000 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> *
> 0372000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> *
> 0772000
>
> A test case submission for fstests follows soon.
>
> Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
> ---
> fs/btrfs/tree-log.c | 108 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 108 insertions(+)
>
> diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
> index 7ac45cf..ac90336 100644
> --- a/fs/btrfs/tree-log.c
> +++ b/fs/btrfs/tree-log.c
> @@ -4203,6 +4203,107 @@ static int btrfs_log_all_xattrs(struct btrfs_trans_handle *trans,
> return 0;
> }
>
> +/*
> + * If the no holes feature is enabled we need to make sure any hole between the
> + * last extent and the i_size of our inode is explicitly marked in the log. This
> + * is to make sure that doing something like:
> + *
> + * 1) create file with 128Kb of data
> + * 2) truncate file to 64Kb
> + * 3) truncate file to 256Kb
> + * 4) fsync file
> + * 5) <crash/power failure>
> + * 6) mount fs and trigger log replay
> + *
> + * Will give us a file with a size of 256Kb, the first 64Kb of data match what
> + * the file had in its first 64Kb of data at step 1 and the last 192Kb of the
> + * file correspond to a hole. The presence of explicit holes in a log tree is
> + * what guarantees that log replay will remove/adjust file extent items in the
> + * fs/subvol tree.
> + *
> + * Here we do not need to care about holes between extents, that is already done
> + * by copy_items(). We also only need to do this in the full sync path, where we
> + * lookup for extents from the fs/subvol tree only. In the fast path case, we
> + * lookup the list of modified extent maps and if any represents a hole, we
> + * insert a corresponding extent representing a hole in the log tree.
> + */
> +static int btrfs_log_trailing_hole(struct btrfs_trans_handle *trans,
> + struct btrfs_root *root,
> + struct inode *inode,
> + struct btrfs_path *path)
> +{
> + int ret;
> + struct btrfs_key key;
> + u64 hole_start;
> + u64 hole_size;
> + struct extent_buffer *leaf;
> + struct btrfs_root *log = root->log_root;
> + const u64 ino = btrfs_ino(inode);
> + const u64 i_size = i_size_read(inode);
> +
> + if (!btrfs_fs_incompat(root->fs_info, NO_HOLES))
> + return 0;
> +
> + key.objectid = ino;
> + key.type = BTRFS_EXTENT_DATA_KEY;
> + key.offset = (u64)-1;
> +
> + ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
> + ASSERT(ret != 0);
> + if (ret < 0)
> + return ret;
> +
> + ASSERT(path->slots[0] > 0);
> + path->slots[0]--;
> + leaf = path->nodes[0];
> + btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
> +
> + if (key.objectid != ino || key.type != BTRFS_EXTENT_DATA_KEY) {
> + /* inode does not have any extents */
> + hole_start = 0;
> + hole_size = i_size;
> + } else {
> + struct btrfs_file_extent_item *extent;
> + u64 len;
> +
> + /*
> + * If there's an extent beyond i_size, an explicit hole was
> + * already inserted by copy_items().
> + */
> + if (key.offset >= i_size)
> + return 0;
> +
> + extent = btrfs_item_ptr(leaf, path->slots[0],
> + struct btrfs_file_extent_item);
> +
> + if (btrfs_file_extent_type(leaf, extent) ==
> + BTRFS_FILE_EXTENT_INLINE) {
> + len = btrfs_file_extent_inline_len(leaf,
> + path->slots[0],
> + extent);
> + ASSERT(len == i_size);
> + return 0;
> + }
> +
> + len = btrfs_file_extent_num_bytes(leaf, extent);
> + /* Last extent goes beyond i_size, no need to log a hole. */
> + if (key.offset + len > i_size)
> + return 0;
> + hole_start = key.offset + len;
> + hole_size = i_size - hole_start;
> + }
> + btrfs_release_path(path);
> +
> + /* Last extent ends at i_size. */
> + if (hole_size == 0)
> + return 0;
> +
> + hole_size = ALIGN(hole_size, root->sectorsize);
> + ret = btrfs_insert_file_extent(trans, log, ino, hole_start, 0, 0,
> + hole_size, 0, hole_size, 0, 0, 0);
> + return ret;
> +}
> +
> /* log a single inode in the tree log.
> * At least one parent directory for this inode must exist in the tree
> * or be logged already.
> @@ -4466,6 +4567,13 @@ next_slot:
> err = btrfs_log_all_xattrs(trans, root, inode, path, dst_path);
> if (err)
> goto out_unlock;
> + if (max_key.type >= BTRFS_EXTENT_DATA_KEY && !fast_search) {
> + btrfs_release_path(path);
> + btrfs_release_path(dst_path);
> + err = btrfs_log_trailing_hole(trans, root, inode, path);
> + if (err)
> + goto out_unlock;
> + }
> log_extents:
> btrfs_release_path(path);
> btrfs_release_path(dst_path);
> --
> 2.1.3
>
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