Re: [PATCH v3] loop: fix no-unmap write-zeroes request behavior

[Eric Sandeen <sandeen@sandeen.net>]

On 10/14/19 10:50 AM, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> 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@oracle.com>
> ---
> v3: refactor into a single fallocate function
> v2: reorganize a little according to hch feedback
> ---
>   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 f6f77eaa7217..ef6e251857c8 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);
cxz ÿbvVBV
> +	case REQ_OP_DISCARD:
> +		return lo_fallocate(lo, rq, pos, FALLOC_FL_PUNCH_HOLE);

I get lost in the twisty passages.  What happens if the filesystem hosting the
backing file doesn't support fallocate, and REQ_OP_DISCARD / REQ_OP_WRITE_ZEROES
returns EOPNOTSUPP - discard is advisory, is it ok to fail REQ_OP_WRITE_ZEROES?
Does something at another layer fall back to writing zeros?

-Eric

>   	case REQ_OP_WRITE:
>   		if (lo->transfer)
>   			return lo_write_transfer(lo, rq, pos);
>