Re: [PATCH 0/4] Fix incorrect splitting logic in btrfs_drop_extent_map_range

[David Sterba <dsterba@suse.cz>]

On Thu, Aug 17, 2023 at 04:57:29PM -0400, Josef Bacik wrote:
> We have been hitting a fair number of warnings in btrfs_drop_extent_map_range
> and in unpin_extent_map in production.  Upon investigation I discovered we were
> splitting improperly when we call btrfs_drop_extent_map_range with skip_pinned.
> This results in invalid extent_maps in the inode's io_tree, which in turn wreaks
> all sorts of havoc, mostly in the form of these WARN_ON()'s.  This took me a
> while to spot so I have a bunch of self-tests that test various functionality of
> btrfs_drop_extent_map_range and btrfs_add_extent_mapping, with one test that
> actual exercises the bug.
> 
> This has been broken for a while, and thankfully is only triggered in certain
> cases with relocation on.  Our environment uses auto relocation heavily which is
> why we hit this reliably, but the incident rate is still relatively low.  The
> bug was introduced over 10 years ago, it probably could be limited to being
> backported to the most recent kernels, basically anytime after Filipe's cleaning
> up of this code.  Thanks,
> 
> Josef
> 
> Josef Bacik (4):
>   btrfs: fix incorrect splitting in btrfs_drop_extent_map_range
>   btrfs: add extent_map tests for dropping with odd layouts
>   btrfs: add a self test for btrfs_add_extent_mapping
>   btrfs: test invalid splitting when skipping pinned drop extent_map

Nice, we have a new record holder, thanks for tracking it down and for
adding the tests. I'll add the patches to misc-next but review is still open.