Re: [PATCH net-next 3/6] mm/page_alloc: use initial zero offset for page_frag_alloc_align()

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2024/1/9 0:25, Alexander Duyck wrote:
> On Mon, Jan 8, 2024 at 12:59 AM Yunsheng Lin <linyunsheng@huawei.com> wrote:

...

> 
>>>
>>> 2. By starting at the end and working toward zero we can use built in
>>> functionality of the CPU to only have to check and see if our result
>>> would be signed rather than having to load two registers with the
>>> values and then compare them which saves us a few cycles. In addition
>>> it saves us from having to read both the size and the offset for every
>>> page.
>>
>> I suppose the above is ok if we only use the page_frag_alloc*() API to
>> allocate memory for skb->data, not for the frag in skb_shinfo(), as by
>> starting at the end and working toward zero, it means we can not do skb
>> coalescing.
>>
>> As page_frag_alloc*() is returning va now, I am assuming most of users
>> is using the API for skb->data, I guess it is ok to drop this patch for
>> now.
>>
>> If we allow page_frag_alloc*() to return struct page, we might need this
>> patch to enable coalescing.
> 
> I would argue this is not the interface for enabling coalescing. This
> is one of the reasons why this is implemented the way it is. When you
> are aligning fragments you aren't going to be able to coalesce the
> frames anyway as the alignment would push the fragments apart.

It seems the alignment requirement is the same for the same user of a page_frag
instance, so the aligning does not seem to be a problem for coalescing?

> .
>