Re: [PATCH net-next v13 04/14] mm: page_frag: add '_va' suffix to page_frag API

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2024/8/19 23:54, Alexander Duyck wrote:

...

>>>>
>>>> "There are three types of API as proposed in this patchset instead of
>>>> two types of API:
>>>> 1. page_frag_alloc_va() returns [va].
>>>> 2. page_frag_alloc_pg() returns [page, offset].
>>>> 3. page_frag_alloc() returns [va] & [page, offset].
>>>>
>>>> You seemed to miss that we need a third naming for the type 3 API.
>>>> Do you see type 3 API as a valid API? if yes, what naming are you
>>>> suggesting for it? if no, why it is not a valid API?"
>>>
>>> I didn't. I just don't see the point in pushing out the existing API
>>> to support that. In reality 2 and 3 are redundant. You probably only
>>> need 3. Like I mentioned earlier you can essentially just pass a
>>
>> If the caller just expect [page, offset], do you expect the caller also
>> type 3 API, which return both [va] and [page, offset]?
>>
>> I am not sure if I understand why you think 2 and 3 are redundant here?
>> If you think 2 and 3 are redundant here, aren't 1 and 3 also redundant
>> as the similar agrument?
> 
> The big difference is the need to return page and offset. Basically to
> support returning page and offset you need to pass at least one value
> as a pointer so you can store the return there.
> 
> The reason why 3 is just a redundant form of 2 is that you will
> normally just be converting from a va to a page and offset so the va
> should already be easily accessible.

I am assuming that by 'easily accessible', you meant the 'va' can be
calculated as below, right?

va = encoded_page_address(encoded_va) +
		(page_frag_cache_page_size(encoded_va) - remaining);

I guess it is easily accessible, but it is not without some overhead
to calculate the 'va' here.

> 
>>> page_frag via pointer to the function. With that you could also look
>>> at just returning a virtual address as well if you insist on having
>>> something that returns all of the above. No point in having 2 and 3 be
>>> seperate functions.
>>
>> Let's be more specific about what are your suggestion here: which way
>> is the prefer way to return the virtual address. It seems there are two
>> options:
>>
>> 1. Return the virtual address by function returning as below:
>> void *page_frag_alloc_bio(struct page_frag_cache *nc, struct bio_vec *bio);
>>
>> 2. Return the virtual address by double pointer as below:
>> int page_frag_alloc_bio(struct page_frag_cache *nc, struct bio_vec *bio,
>>                         void **va);
> 
> I was thinking more of option 1. Basically this is a superset of
> page_frag_alloc_va that is also returning the page and offset via a
> page frag. However instead of bio_vec I would be good with "struct
> page_frag *" being the value passed to the function to play the role
> of container. Basically the big difference between 1 and 2/3 if I am
> not mistaken is the fact that for 1 you pass the size, whereas with
> 2/3 you are peeling off the page frag from the larger page frag cache

Let's be clear here: The callers just expecting [page, offset] also need
to call type 3 API, which return both [va] and [page, offset]? and it
is ok to ignore the overhead of calculating the 'va' for those kinds
of callers just because we don't want to do the renaming for a existing
API and can't come up with good naming for that?

> after the fact via a commit type action.

Just be clear here, there is no commit type action for some subtype of
type 2/3 API.

For example, for type 2 API in this patchset, it has below subtypes:

subtype 1: it does not need a commit type action, it just return
           [page, offset] instead of page_frag_alloc_va() returning [va],
           and it does not return the allocated fragsz back to the caller
           as page_frag_alloc_va() does not too:
struct page *page_frag_alloc_pg(struct page_frag_cache *nc,
                                unsigned int *offset, unsigned int fragsz,
                                gfp_t gfp)

subtype 2: it does need a commit type action, and @fragsz is returned to
           the caller and caller used that to commit how much fragsz to
           commit.
struct page *page_frag_alloc_pg_prepare(struct page_frag_cache *nc,
                                        unsigned int *offset,
                                        unsigned int *fragsz, gfp_t gfp)

Do you see subtype 1 as valid API? If no, why?
If yes, do you also expect the caller to use "struct page_frag *" as the
container? If yes, what is the caller expected to do with the size field in
"struct page_frag *" from API perspective? Just ignore it?