[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