Re: [PATCH v4] mm: introduce reference pages

[John Hubbard <jhubbard@nvidia.com>]

On 6/29/21 4:58 AM, Matthew Wilcox wrote:
> On Tue, Jun 29, 2021 at 12:19:22AM -0700, John Hubbard wrote:
>>> +SYSCALL_DEFINE2(refpage_create, const void *__user, content, unsigned long,
>>> +		flags)
>>
>>  From the API discussion (and using a simpler syntax to illustrate this), it
>> seems like the following would be close:
>>
>> enum content_type {
>> 	BYTE_PATTERN,
>> 	FOUR_BYTE_PATTERN,
>> 	...
>> 	FULL_4KB_PAGE
>> };
>>
>> int refpage_create(const void *__user content, enum content_type, unsigned long flags);
>>
>> ...and if content_type == BYTE_PATTERN, then content is a pointer to just one byte of
>> data, and so forth for the other enum values.
> 
> That seems a little more complicated and non-extensible.

This is true. I wasn't sure I liked it all that much, either, when I wrote
it. haha.

> 
> int refpage_create(const void *__user content, unsigned int size,
> 		unsigned long flags);
> 

That does seem better. The key is to have at least one more parameter.

Actually I forgot to include pattern data. In both of the approaches above,
flags is probably used for that, but if we already know that patterns
are being passed, then how about add a "pattern" arg? I think it's
good to leave a little room for flexibility and future extensions:

int refpage_create(const void *__user content, unsigned int size,
		unsigned long pattern, unsigned long flags);


thanks,
-- 
John Hubbard
NVIDIA