Re: [PATCH] Btrfs: fix data bytes_may_use underflow with fallocate due to failed quota reserve

[Filipe Manana <fdmanana@gmail.com>]

On Mon, Jan 14, 2019 at 3:26 AM robbieko <robbieko@synology.com> wrote:
>
> From: Robbie Ko <robbieko@synology.com>
>
> When doing fallocate, we first add the range to the reserve_list
> and then reserve the qutoa.

qutoa -> quota

> If quota reservation fails, we'll release all reserved parts of
> reserve_list.
> However, cur_offset doen't update to indicate that this range is

"doen't update" -> is not updated to indicate ...

> already been inserted into the list.
> Therefore, the same range is freed twice.
> One at list_for_each_entry loop, and the other at the end of the

One -> once
and the other at -> and once at

> function.
> This will result in WARN_ON on bytes_may_use when we free the
> remaining space.
>
> Fixes: ("btrfs: update btrfs_space_info's bytes_may_use timely")

Missing commit id.

> Signed-off-by: Robbie Ko <robbieko@synology.com>
> ---
>  fs/btrfs/file.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index d38dc8c..43c6c8a 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -3132,6 +3132,7 @@ static long btrfs_fallocate(struct file *file, int mode,
>                         ret = btrfs_qgroup_reserve_data(inode, &data_reserved,
>                                         cur_offset, last_byte - cur_offset);
>                         if (ret < 0) {
> +                               cur_offset = last_byte;

This will fix the problem if there's only one range, or we happen to
be very lucky and cur_offset
matches exactly the middle of the entire range passed to fallocate()
(in case the range covers
a sparse region of the file, a region with holes and extents).

At the end, under the 'out' label we have a call to:

   btrfs_free_reserved_data_space(inode, data_reserved, alloc_start,
alloc_end - cur_offset);

That doesn't fell right - the start offset, third argument, should be
cur_offset. Everything from
alloc_start to cur_offset was freed by the list_for_each_entry_safe_loop.





>                                 free_extent_map(em);
>                                 break;
>                         }
> --
> 1.9.1
>


-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”