Re: [PATCH] loop: change to punch hole if zero range is not supported

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Jul 01, 2020 at 09:54:01PM -0400, Shijie Luo wrote:
> We found a problem when excuting these operations.
> 
> $ cd /tmp
> $ qemu-img create -f raw test.img 10G
> $ mknod -m 0660 /dev/loop0 b 7 0
> $ losetup /dev/loop0 test.img
> $ mkfs /dev/loop0
> 
> Here is the error message.
> 
> [  142.364823] blk_update_request: operation not supported error,
>  dev loop0, sector 20971392 op 0x9:(WRITE_ZEROES) flags 0x1000800
>  phys_seg 0 prio class 0
> [  142.371823] blk_update_request: operation not supported error,
>  dev loop0, sector 5144 op 0x9:(WRITE_ZEROES) flags 0x1000800
>  phys_seg 0 prio class 0
> 
> The problem is that not all filesystem support zero range (eg, tmpfs), if
>  filesystem doesn 't support zero range, change to punch hole to fix it.

NAK, ZERO_RANGE requires[1] that "Within the specified range, blocks are
preallocated for the regions that span the holes in the file."
PUNCH_HOLE has the opposite effect, and does not qualify as a
replacement.

--D

[1] https://man7.org/linux/man-pages/man2/fallocate.2.html

> Fixes: efcfec579f613 ("loop: fix no-unmap write-zeroes request behavior")
> Signed-off-by: Shijie Luo <luoshijie1@huawei.com>
> ---
>  drivers/block/loop.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index c33bbbfd1bd9..504e658adcaf 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -450,6 +450,13 @@ static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos,
>  	}
>  
>  	ret = file->f_op->fallocate(file, mode, pos, blk_rq_bytes(rq));
> +
> +	if ((ret == -EOPNOTSUPP) && (mode & FALLOC_FL_ZERO_RANGE)) {
> +		mode &= ~FALLOC_FL_ZERO_RANGE;
> +		mode |= FALLOC_FL_PUNCH_HOLE;
> +		ret = file->f_op->fallocate(file, mode, pos, blk_rq_bytes(rq));
> +	}
> +
>  	if (unlikely(ret && ret != -EINVAL && ret != -EOPNOTSUPP))
>  		ret = -EIO;
>   out:
> -- 
> 2.19.1
>