[PATCH v20 04/12] block: add emulation for copy

[Christoph Hellwig]

On Mon, May 20, 2024 at 03:50:17PM +0530, Nitesh Shetty wrote:
> For the devices which does not support copy, copy emulation is added.
> It is required for in-kernel users like fabrics, where file descriptor is
> not available and hence they can't use copy_file_range.
> Copy-emulation is implemented by reading from source into memory and
> writing to the corresponding destination.
> At present in kernel user of emulation is fabrics.

I still don't see the point of offering this in the block layer,
at least in this form.  Caller usually can pre-allocate a buffer
if they need regular copies instead of doing constant allocation
and freeing which puts a lot of stress on the page allocator.

> +static void *blkdev_copy_alloc_buf(ssize_t req_size, ssize_t *alloc_size,
> +				   gfp_t gfp)
> +{
> +	int min_size = PAGE_SIZE;
> +	char *buf;
> +
> +	while (req_size >= min_size) {
> +		buf = kvmalloc(req_size, gfp);
> +		if (buf) {
> +			*alloc_size = req_size;
> +			return buf;
> +		}
> +		req_size >>= 1;
> +	}
> +
> +	return NULL;

And requiring a kernel mapping for data is is never used through the
kernel mapping is pretty silly as well.