[Devel] [PATCH RH8] loop: fix no-unmap write-zeroes request behavior

[Konstantin Khorenko]

Dropping this as already applied by RedHat:

* Thu Apr 16 2020 Frantisek Hrbata <fhrbata at redhat.com> [4.18.0-193.9.el8]
- [block] loop: fix no-unmap write-zeroes request behavior (Ming Lei) [1798919]

--
Best regards,

Konstantin Khorenko,
Virtuozzo Linux Kernel Team

On 11/17/2020 01:21 PM, Kirill Tkhai wrote:
> From: Darrick J. Wong <darrick.wong at oracle.com>
>
> ms commit efcfec579f61
>
> Currently, if the loop device receives a WRITE_ZEROES request, it asks
> the underlying filesystem to punch out the range.  This behavior is
> correct if unmapping is allowed.  However, a NOUNMAP request means that
> the caller doesn't want us to free the storage backing the range, so
> punching out the range is incorrect behavior.
>
> To satisfy a NOUNMAP | WRITE_ZEROES request, loop should ask the
> underlying filesystem to FALLOC_FL_ZERO_RANGE, which is (according to
> the fallocate documentation) required to ensure that the entire range is
> backed by real storage, which suffices for our purposes.
>
> Fixes: 19372e2769179dd ("loop: implement REQ_OP_WRITE_ZEROES")
> Signed-off-by: Darrick J. Wong <darrick.wong at oracle.com>
> Reviewed-by: Christoph Hellwig <hch at lst.de>
> Signed-off-by: Jens Axboe <axboe at kernel.dk>
> Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
> ---
>  drivers/block/loop.c |   26 ++++++++++++++++++--------
>  1 file changed, 18 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 5b124d57b9c0..6308dabc253f 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -417,18 +417,20 @@ static int lo_read_transfer(struct loop_device *lo, struct request *rq,
>  	return ret;
>  }
>
> -static int lo_discard(struct loop_device *lo, struct request *rq, loff_t pos)
> +static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos,
> +			int mode)
>  {
>  	/*
> -	 * We use punch hole to reclaim the free space used by the
> -	 * image a.k.a. discard. However we do not support discard if
> -	 * encryption is enabled, because it may give an attacker
> -	 * useful information.
> +	 * We use fallocate to manipulate the space mappings used by the image
> +	 * a.k.a. discard/zerorange. However we do not support this if
> +	 * encryption is enabled, because it may give an attacker useful
> +	 * information.
>  	 */
>  	struct file *file = lo->lo_backing_file;
> -	int mode = FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE;
>  	int ret;
>
> +	mode |= FALLOC_FL_KEEP_SIZE;
> +
>  	if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
>  		ret = -EOPNOTSUPP;
>  		goto out;
> @@ -596,9 +598,17 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq)
>  	switch (req_op(rq)) {
>  	case REQ_OP_FLUSH:
>  		return lo_req_flush(lo, rq);
> -	case REQ_OP_DISCARD:
>  	case REQ_OP_WRITE_ZEROES:
> -		return lo_discard(lo, rq, pos);
> +		/*
> +		 * If the caller doesn't want deallocation, call zeroout to
> +		 * write zeroes the range.  Otherwise, punch them out.
> +		 */
> +		return lo_fallocate(lo, rq, pos,
> +			(rq->cmd_flags & REQ_NOUNMAP) ?
> +				FALLOC_FL_ZERO_RANGE :
> +				FALLOC_FL_PUNCH_HOLE);
> +	case REQ_OP_DISCARD:
> +		return lo_fallocate(lo, rq, pos, FALLOC_FL_PUNCH_HOLE);
>  	case REQ_OP_WRITE:
>  		if (lo->transfer)
>  			return lo_write_transfer(lo, rq, pos);
>
>
> .
>