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After deleteing snapshots, we may be left with a snapshot tree where some nodes only have one child, and we have a linear chain. Interior snapshot nodes are never used directly (i.e. they never have subvolumes that point to them), they are only referered to by child snapshot nodes - hence, they are redundant. The existing code talks about redundant snapshot nodes as forming and equivalence class; i.e. nodes for which snapshot_t->equiv is equal. In a given equivalence class, we only ever need a single key at a given position - i.e. multiple versions with different snapshot fields are redundant. The existing snapshot cleanup code deletes these redundant keys, but not redundant nodes. It turns out this is buggy, because we assume that after snapshot deletion finishes we should only have a single key per equivalence class, but the btree update path doesn't preserve this - overwriting keys in old snapshots doesn't check for the equivalence class being equal, and thus we can end up with duplicate keys in the same equivalence class and fsck complaining about snapshot deletion not having run correctly. The equivalence class notion has been leaking out of the core snapshots code and into too much other code, i.e. fsck, so this patch takes a different approach: snapshot deletion now moves keys to the node in an equivalence class being kept (the leafiest node) and then deletes the redundant nodes in the equivalance class. Some work has to be done to correctly delete interior snapshot nodes; snapshot node depth and skiplist fields for descendent nodes have to be fixed. 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 Restructured Text 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.