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

Christoph Hellwig
2024-06-01T06:18:44Z

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.