Re: [PATCH v13 3/9] block: add emulation for copy

[Nitesh Shetty]

On 23/07/20 09:50AM, Christoph Hellwig wrote:
> > +static void *blkdev_copy_alloc_buf(sector_t req_size, sector_t *alloc_size,
> > +		gfp_t gfp_mask)
> > +{
> > +	int min_size = PAGE_SIZE;
> > +	void *buf;
> > +
> > +	while (req_size >= min_size) {
> > +		buf = kvmalloc(req_size, gfp_mask);
> > +		if (buf) {
> > +			*alloc_size = req_size;
> > +			return buf;
> > +		}
> > +		/* retry half the requested size */
> > +		req_size >>= 1;
> > +	}
> > +
> > +	return NULL;
>
> Is there any good reason for using vmalloc instead of a bunch
> of distcontiguous pages?

kvmalloc seemed convenient for the purpose. 
We will need to call alloc_page in a loop to guarantee discontigous pages. 
Do you prefer that over kvmalloc?

> > +		ctx = kzalloc(sizeof(struct copy_ctx), gfp_mask);
> > +		if (!ctx)
> > +			goto err_ctx;
>
> I'd suspect it would be better to just allocte a single buffer and
> only have a single outstanding copy.  That will reduce the bandwith
> you can theoretically get, but copies tend to be background operations
> anyway.  It will reduce the required memory, and thus the chance for
> this operation to fail on a loaded system.  It will also dramatically
> reduce the effect on memory managment.

Next version will have that change.

Thank You,
Nitesh Shetty