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We have been using the following check if (generation <= root->root_key.offset) to make decisions about whether or not to visit a node during snapshot delete. This is because for normal subvolumes this is set to 0, and for snapshots it's set to the creation generation. The idea being that if the generation of the node is less than or equal to our creation generation then we don't need to visit that node, because it doesn't belong to us, we can simply drop our reference and move on. However reloc roots don't have their generation stored in root->root_key.offset, instead that is the objectid of their corresponding fs root. This means we can incorrectly not walk into nodes that need to be dropped when deleting a reloc root. There are a variety of consequences to making the wrong choice in two distinct areas. visit_node_for_delete() 1. False positive. We think we are newer than the block when we really aren't. We don't visit the node and drop our reference to the node and carry on. This would result in leaked space. 2. False negative. We do decide to walk down into a block that we should have just dropped our reference to. However this means that the child node will have refs > 1, so we will switch to UPDATE_BACKREF, and then the subsequent walk_down_proc() will notice that btrfs_header_owner(node) != root->root_key.objectid and it'll break out of the loop, and then walk_up_proc() will drop our reference, so this appears to be ok. do_walk_down() 1. False positive. We are in UPDATE_BACKREF and incorrectly decide that we are done and don't need to update the backref for our lower nodes. This is another case that simply won't happen with relocation, as we only have to do UPDATE_BACKREF if the node below us was shared and didn't have FULL_BACKREF set, and since we don't own that node because we're a reloc root we actually won't end up in this case. 2. False negative. Again this is tricky because as described above, we simply wouldn't be here from relocation, because we don't own any of the nodes because we never set btrfs_header_owner() to the reloc root objectid, and we always use FULL_BACKREF, we never actually need to set FULL_BACKREF on any children. Having spent a lot of time stressing relocation/snapshot delete recently I've not seen this pop in practice. But this is objectively incorrect, so fix this to get the correct starting generation based on the root we're dropping to keep me from thinking there's a problem here. CC: stable@vger.kernel.org Reviewed-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: Josef Bacik <josef@toxicpanda.com> Signed-off-by: David Sterba <dsterba@suse.com> |
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| drivers | ||
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| init | ||
| io_uring | ||
| ipc | ||
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| lib | ||
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| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
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| README | ||
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.