On Wed, Nov 09, 2016 at 03:26:50PM -0800, Omar Sandoval wrote:
> From: Omar Sandoval <osan...@fb.com>
>
> My QEMU VM was seeing inexplicable I/O errors that I tracked down to
> errors coming from the qcow2 virtual drive in the host system. The qcow2
> file is a nocow file on my Btrfs drive, which QEMU opens with O_DIRECT.
> Every once in awhile, pread() or pwrite() would return EEXIST, which
> makes no sense. This turned out to be a bug in btrfs_get_extent().
>
> Commit 8dff9c853410 ("Btrfs: deal with duplciates during extent_map
> insertion in btrfs_get_extent") fixed a case in btrfs_get_extent() where
> two threads race on adding the same extent map to an inode's extent map
> tree. However, if the added em is merged with an adjacent em in the
> extent tree, then we'll end up with an existing extent that is not
> identical to but instead encompasses the extent we tried to add. When we
> call merge_extent_mapping() to find the nonoverlapping part of the new
> em, the arithmetic overflows because there is no such thing. We then end
> up trying to add a bogus em to the em_tree, which results in a EEXIST
> that can bubble all the way up to userspace.
Is this possibly the same bug that Liu Bo fixes in
https://patchwork.kernel.org/patch/9413129/ ?
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