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When we are checking if we need to delay the rename operation for an
inode we not checking if a parent inode that exists in the send and
parent snapshots is really the same inode or not, that is, we are not
comparing the generation number of the parent inode in the send and
parent snapshots. Not only this results in unnecessarily delaying a
rename operation but also can later on make us generate an incorrect
name for a new inode in the send snapshot that has the same number
as another inode in the parent snapshot but a different generation.
Here follows an example where this happens.
Parent snapshot:
. (ino 256, gen 3)
|--- dir258/ (ino 258, gen 7)
| |--- dir257/ (ino 257, gen 7)
|
|--- dir259/ (ino 259, gen 7)
Send snapshot:
. (ino 256, gen 3)
|--- file258 (ino 258, gen 10)
|
|--- new_dir259/ (ino 259, gen 10)
|--- dir257/ (ino 257, gen 7)
The following steps happen when computing the incremental send stream:
1) When processing inode 257, its new parent is created using its orphan
name (o257-21-0), and the rename operation for inode 257 is delayed
because its new parent (inode 259) was not yet processed - this
decision to delay the rename operation does not make much sense
because the inode 259 in the send snapshot is a new inode, it's not
the same as inode 259 in the parent snapshot.
2) When processing inode 258 we end up delaying its rmdir operation,
because inode 257 was not yet renamed (moved away from the directory
inode 258 represents). We also create the new inode 258 using its
orphan name "o258-10-0", then rename it to its final name of "file258"
and then issue a truncate operation for it. However this truncate
operation contains an incorrect name, which corresponds to the orphan
name and not to the final name, which makes the receiver fail. This
happens because when we attempt to compute the inode's current name
we verify that there's another inode with the same number (258) that
has its rmdir operation pending and because of that we generate an
orphan name for the new inode 258 (we do this in the function
get_cur_path()).
Fix this by not delayed the rename operation of an inode if it has parents
with the same number but different generations in both snapshots.
The following steps reproduce this example scenario.
$ mkfs.btrfs -f /dev/sdb
$ mount /dev/sdb /mnt
$ mkdir /mnt/dir257
$ mkdir /mnt/dir258
$ mkdir /mnt/dir259
$ mv /mnt/dir257 /mnt/dir258/dir257
$ btrfs subvolume snapshot -r /mnt /mnt/snap1
$ mv /mnt/dir258/dir257 /mnt/dir257
$ rmdir /mnt/dir258
$ rmdir /mnt/dir259
# Remount the filesystem so that the next created inodes will have the
# numbers 258 and 259. This is because when a filesystem is mounted,
# btrfs sets the subvolume's inode counter to a value corresponding to
# the highest inode number in the subvolume plus 1. This inode counter
# is used to assign a unique number to each new inode and it's
# incremented by 1 after very inode creation.
# Note: we unmount and then mount instead of doing a mount with
# "-o remount" because otherwise the inode counter remains at value 260.
$ umount /mnt
$ mount /dev/sdb /mnt
$ touch /mnt/file258
$ mkdir /mnt/new_dir259
$ mv /mnt/dir257 /mnt/new_dir259/dir257
$ btrfs subvolume snapshot -r /mnt /mnt/snap2
$ btrfs send /mnt/snap1 -f /tmp/1.snap
$ btrfs send -p /mnt/snap1 /mnt/snap2 -f /tmp/2.snap
$ umount /mnt
$ mkfs.btrfs -f /dev/sdc
$ mount /dev/sdc /mnt
$ btrfs receive /mnt -f /tmo/1.snap
$ btrfs receive /mnt -f /tmo/2.snap -vv
receiving snapshot mysnap2 uuid=e059b6d1-7f55-f140-8d7c-9a3039d23c97, ctransid=10 parent_uuid=77e98cb6-8762-814f-9e05-e8ba877fc0b0, parent_ctransid=7
utimes
mkdir o259-10-0
rename dir258 -> o258-7-0
utimes
mkfile o258-10-0
rename o258-10-0 -> file258
utimes
truncate o258-10-0 size=0
ERROR: truncate o258-10-0 failed: No such file or directory
Reported-by: Robbie Ko <robbieko@synology.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.