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docs: update offline parent pointer repair strategy

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Now update how xfs_repair checks and repairs parent pointer info.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
This commit is contained in:
Darrick J. Wong 2024-04-15 14:55:09 -07:00
parent 5220727ce8
commit c91fe20e5a
1 changed files with 60 additions and 21 deletions
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81
Documentation/filesystems/xfs/xfs-online-fsck-design.rst
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@ -4675,26 +4675,56 @@ files are erased long before directory tree connectivity checks are performed.
Parent pointer checks are therefore a second pass to be added to the existing
connectivity checks:
1. After the set of surviving files has been established (i.e. phase 6),
1. After the set of surviving files has been established (phase 6),
walk the surviving directories of each AG in the filesystem.
This is already performed as part of the connectivity checks.
2. For each directory entry found, record the name in an xfblob, and store
``(child_ag_inum, parent_inum, parent_gen, dirent_pos)`` tuples in a
per-AG in-memory slab.
2. For each directory entry found,
a. If the name has already been stored in the xfblob, then use that cookie
and skip the next step.
b. Otherwise, record the name in an xfblob, and remember the xfblob cookie.
Unique mappings are critical for
1. Deduplicating names to reduce memory usage, and
2. Creating a stable sort key for the parent pointer indexes so that the
parent pointer validation described below will work.
c. Store ``(child_ag_inum, parent_inum, parent_gen, name_hash, name_len,
name_cookie)`` tuples in a per-AG in-memory slab. The ``name_hash``
referenced in this section is the regular directory entry name hash, not
the specialized one used for parent pointer xattrs.
3. For each AG in the filesystem,
a. Sort the per-AG tuples in order of child_ag_inum, parent_inum, and
dirent_pos.
a. Sort the per-AG tuple set in order of ``child_ag_inum``, ``parent_inum``,
``name_hash``, and ``name_cookie``.
Having a single ``name_cookie`` for each ``name`` is critical for
handling the uncommon case of a directory containing multiple hardlinks
to the same file where all the names hash to the same value.
b. For each inode in the AG,
1. Scan the inode for parent pointers.
Record the names in a per-file xfblob, and store ``(parent_inum,
parent_gen, dirent_pos)`` tuples in a per-file slab.
For each parent pointer found,
2. Sort the per-file tuples in order of parent_inum, and dirent_pos.
a. Validate the ondisk parent pointer.
If validation fails, move on to the next parent pointer in the
file.
b. If the name has already been stored in the xfblob, then use that
cookie and skip the next step.
c. Record the name in a per-file xfblob, and remember the xfblob
cookie.
d. Store ``(parent_inum, parent_gen, name_hash, name_len,
name_cookie)`` tuples in a per-file slab.
2. Sort the per-file tuples in order of ``parent_inum``, ``name_hash``,
and ``name_cookie``.
3. Position one slab cursor at the start of the inode's records in the
per-AG tuple slab.
@ -4703,28 +4733,37 @@ connectivity checks:
4. Position a second slab cursor at the start of the per-file tuple slab.
5. Iterate the two cursors in lockstep, comparing the parent_ino and
dirent_pos fields of the records under each cursor.
5. Iterate the two cursors in lockstep, comparing the ``parent_ino``,
``name_hash``, and ``name_cookie`` fields of the records under each
cursor:
a. Tuples in the per-AG list but not the per-file list are missing and
need to be written to the inode.
a. If the per-AG cursor is at a lower point in the keyspace than the
per-file cursor, then the per-AG cursor points to a missing parent
pointer.
Add the parent pointer to the inode and advance the per-AG
cursor.
b. Tuples in the per-file list but not the per-AG list are dangling
and need to be removed from the inode.
b. If the per-file cursor is at a lower point in the keyspace than
the per-AG cursor, then the per-file cursor points to a dangling
parent pointer.
Remove the parent pointer from the inode and advance the per-file
cursor.
c. For tuples in both lists, update the parent_gen and name components
of the parent pointer if necessary.
c. Otherwise, both cursors point at the same parent pointer.
Update the parent_gen component if necessary.
Advance both cursors.
4. Move on to examining link counts, as we do today.
The proposed patchset is the
`offline parent pointers repair
<https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfsprogs-dev.git/log/?h=pptrs-repair>`_
<https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfsprogs-dev.git/log/?h=pptrs-fsck>`_
series.
Rebuilding directories from parent pointers in offline repair is very
challenging because it currently uses a single-pass scan of the filesystem
during phase 3 to decide which files are corrupt enough to be zapped.
Rebuilding directories from parent pointers in offline repair would be very
challenging because xfs_repair currently uses two single-pass scans of the
filesystem during phases 3 and 4 to decide which files are corrupt enough to be
zapped.
This scan would have to be converted into a multi-pass scan:
1. The first pass of the scan zaps corrupt inodes, forks, and attributes