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Add a mount option for rewinding the journal, bringing the entire filesystem to where it was at a previous point in time. This is for extreme disaster recovery scenarios - it's not intended as an undelete operation. The option takes a journal sequence number; the desired sequence number can be determined with 'bcachefs list_journal' Caveats: - The 'journal_transaction_names' option must have been enabled (it's on by default). The option controls emitting of extra debug info in the journal, so we can see what individual transactions were doing; It also enables journalling of keys being overwritten, which is what we rely on here. - A full fsck run will be automatically triggered since alloc info will be inconsistent. Only leaf node updates to non-alloc btrees are rewound, since rewinding interior btree updates isn't possible or desirable. - We can't do anything about data that was deleted and overwritten. Lots of metadata updates after the point in time we're rewinding to shouldn't cause a problem, since we segragate data and metadata allocations (this is in order to make repair by btree node scan practical on larger filesystems; there's a small 64-bit per device bitmap in the superblock of device ranges with btree nodes, and we try to keep this small). However, having discards enabled will cause problems, since buckets are discarded as soon as they become empty (this is why we don't implement fstrim: we don't need it). Hopefully, this feature will be a one-off thing that's never used again: this was implemented for recovering from the "vfs i_nlink 0 -> subvol deletion" bug, and that bug was unusually disastrous and additional safeguards have since been implemented. But if it does turn out that we need this more in the future, I'll have to implement an option so that empty buckets aren't discarded immediately - lagging by perhaps 1% of device capacity. Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
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.