Re: [PATCH] Btrfs: deal with existing encompassing extent map in btrfs_get_extent()

["Holger Hoffstätte" <holger@applied-asynchrony.com>]

On 11/10/16 00:26, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@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.
> 
> Fix it by extending the identical extent map special case.
> 
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
> Applies to 4.9-rc4.
> 
> Here [1] is a reproducer for this bug that doesn't involve firing up a
> QEMU VM. Also, a big shoutout to BCC [2] and BPF for making it possible
> to debug this on my laptop without compiling a custom kernel and
> rebooting just to add printks [3].
> 
> 1: https://gist.github.com/osandov/d08aabe5d4dec15517e9fde17012fd3b

I can't really make this reproducer fail. It builds and runs fine, but just
exits with no messages (other than the one about drop_caches in dmesg).
It creates the 1MB file and always returns 0. Ideas?

-h