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Btree nodes are log structured; thus, we need to emit whiteouts when we're deleting a key that's been written out to disk. k->needs_whiteout tracks whether a key will need a whiteout when it's deleted, and this requires some careful handling; e.g. the key we're deleting may not have been written out to disk, but it may have overwritten a key that was - thus we need to carry this flag around on overwrites. Invariants: There may be multiple key for the same position in a given node (because of overwrites), but only one of them will be a live (non deleted) key, and only one key for a given position will have the needs_whiteout flag set. Additionally, we don't want to carry around whiteouts that need to be written in the main searchable part of a btree node - btree_iter_peek() will have to skip past them, and this can lead to an O(n^2) issues when doing sequential deletions (e.g. inode rm/truncate). So there's a separate region in the btree node buffer for unwritten whiteouts; these are merge sorted with the rest of the keys we're writing in the btree node write path. The unwritten whiteouts was a later optimization that bch2_sort_keys() didn't take into account; the unwritten whiteouts area means that we never have deleted keys with needs_whiteout set in the main searchable part of a btree node. That means we can simplify and optimize some sort paths, and eliminate an assertion that syzbot found: - Unless we're in the btree node write path, it's always ok to drop whiteouts when sorting - When sorting for a btree node write, we drop the whiteout if it's not from the unwritten whiteouts area, or if it's overwritten by a real key at the same position. This completely eliminates some tricky logic for propagating the needs_whiteout flag: syzbot was able to hit the assertion that checked that there shouldn't be more than one key at the same pos with needs_whiteout set, likely due to a combination of flipping on needs_whiteout on all written keys (they need whiteouts if overwritten), combined with not always dropping unneeded whiteouts, and the tricky logic in the sort path for preserving needs_whiteout that wasn't really needed. 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.