Re: [PATCH] block: fallocate: avoid false positive on collision detection

[Jan Kara]

 On Thu 07-01-21 14:40:22, Maxim Levitsky wrote:
> Align start and end on page boundaries before calling
> invalidate_inode_pages2_range.
> 
> This might allow us to miss a collision if the write and the discard were done
> to the same page and do overlap but it is still better than returning -EBUSY
> if those writes didn't overlap.
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Thanks for getting back to this and I'm sorry I didn't get to this earlier
myself! I actually think the fix should be different as we discussed with
Darrick. Attached patch should fix the issue for you (I'll also post it
formally for inclusion).

								Honza

> ---
>  fs/block_dev.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 9e84b1928b94..97f0d16661b5 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1970,6 +1970,7 @@ static long blkdev_fallocate(struct file *file, int mode, loff_t start,
>  	loff_t end = start + len - 1;
>  	loff_t isize;
>  	int error;
> +	pgoff_t invalidate_first_page, invalidate_last_page;
>  
>  	/* Fail if we don't recognize the flags. */
>  	if (mode & ~BLKDEV_FALLOC_FL_SUPPORTED)
> @@ -2020,12 +2021,23 @@ static long blkdev_fallocate(struct file *file, int mode, loff_t start,
>  
>  	/*
>  	 * Invalidate again; if someone wandered in and dirtied a page,
> -	 * the caller will be given -EBUSY.  The third argument is
> -	 * inclusive, so the rounding here is safe.
> +	 * the caller will be given -EBUSY.
> +	 *
> +	 * If the start/end of the range is not page aligned, exclude the
> +	 * non aligned regions to avoid false positives.
>  	 */
> +	invalidate_first_page = DIV_ROUND_UP(start, PAGE_SIZE);
> +	invalidate_last_page = end >> PAGE_SHIFT;
> +
> +	if ((end + 1) & PAGE_MASK)
> +		invalidate_last_page--;
> +
> +	if (invalidate_last_page < invalidate_first_page)
> +		return 0;
> +
>  	return invalidate_inode_pages2_range(bdev->bd_inode->i_mapping,
> -					     start >> PAGE_SHIFT,
> -					     end >> PAGE_SHIFT);
> +					     invalidate_first_page,
> +					     invalidate_last_page);
>  }
>  
>  const struct file_operations def_blk_fops = {
> -- 
> 2.26.2
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR