Re: [PATCH V5 13/13] Btrfs: Return valid delalloc range when the page does not have PG_Dirty flag set or has been invalidated

[Josef Bacik <jbacik@fb.com>]

On 09/30/2015 06:28 AM, Chandan Rajendra wrote:
> The following issue was observed when running generic/095 test on
> subpagesize-blocksize patchset.
>
> Assume that we are trying to write a dirty page that is mapping file offset
> range [159744, 163839].
>
> writepage_delalloc()
>    find_lock_delalloc_range(*start = 159744, *end = 0)
>      find_delalloc_range()
>        Returns range [X, Y] where (X > 163839)
>      lock_delalloc_pages()
>        One of the pages in range [X, Y] has dirty flag cleared;
>        Loop once more restricting the delalloc range to span only
>        PAGE_CACHE_SIZE bytes;
>      find_delalloc_range()
>        Returns range [356352, 360447];
>      lock_delalloc_pages()
>        The page [356352, 360447] has dirty flag cleared;
>      Returns with *start = 159744 and *end = 0;
>    *start = *end + 1;
>    find_lock_delalloc_range(*start = 1, *end = 0)
>      Finds and returns delalloc range [1, 12288];
>    cow_file_range()
>      Clears delalloc range [1, 12288]
>      Create ordered extent for range [1, 12288]
>
> The ordered extent thus created above breaks the rule that extents have to be
> aligned to the filesystem's block size.
>
> In cases where lock_delalloc_pages() fails (either due to PG_dirty flag being
> cleared or the page no longer being a member of the inode's page cache), this
> patch sets and returns the delalloc range that was found by
> find_delalloc_range().
>
> Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>

Reviewed-by: Josef Bacik <jbacik@fb.com>

Thanks,

Josef