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.
I don't get how this could happen(even after reading Commit
8dff9c853410), btrfs_get_extent in direct_IO is protected by
lock_extent_direct, the assumption is that a racy thread should be
blocked by lock_extent_direct and when it gets the lock, it finds the
just-inserted em when going into btrfs_get_extent if its offset is
within [em->start, extent_map_end(em)].
I think we may also need to figure out why the above doesn't work as
expected besides fixing another special case.
Thanks,
-liubo
>
> Fix it by extending the identical extent map special case.
>
> Signed-off-by: Omar Sandoval <osan...@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
> 2: https://github.com/iovisor/bcc
> 3: https://gist.github.com/osandov/eb1db868ce10c3af9e00b90f3a65bf9f
>
> fs/btrfs/inode.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 2b790bd..e5cf589 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -7049,11 +7049,11 @@ struct extent_map *btrfs_get_extent(struct inode
> *inode, struct page *page,
> * extent causing the -EEXIST.
> */
> if (existing->start == em->start &&
> - extent_map_end(existing) == extent_map_end(em) &&
> + extent_map_end(existing) >= extent_map_end(em) &&
> em->block_start == existing->block_start) {
> /*
> - * these two extents are the same, it happens
> - * with inlines especially
> + * The existing extent map already encompasses the
> + * entire extent map we tried to add.
> */
> free_extent_map(em);
> em = existing;
> --
> 2.10.2
>
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