Re: [PATCH net-next v4 1/4] mm: add a signature in struct page

Yunsheng Lin
2021-05-13T03:25:40Z

On 2021/5/13 10:35, Matthew Wilcox wrote:
> On Thu, May 13, 2021 at 10:15:26AM +0800, Yunsheng Lin wrote:
>> On 2021/5/12 23:57, Matthew Wilcox wrote:
>>> You'll need something like this because of the current use of
>>> page->index to mean "pfmemalloc".
>>>
>>> @@ -1682,12 +1684,12 @@ static inline bool page_is_pfmemalloc(const struct page *page)
>>> */
>>> static inline void set_page_pfmemalloc(struct page *page)
>>> {
>>> - page->index = -1UL;
>>> + page->compound_head = 2;
>>
>> Is there any reason why not use "page->compound_head |= 2"? as
>> corresponding to the "page->compound_head & 2" in the above
>> page_is_pfmemalloc()?
>>
>> Also, this may mean we need to make sure to pass head page or
>> base page to set_page_pfmemalloc() if using
>> "page->compound_head = 2", because it clears the bit 0 and head
>> page ptr for tail page too, right?
>
> I think what you're missing here is that this page is freshly allocated.
> This is information being passed from the page allocator to any user
> who cares to look at it. By definition, it's set on the head/base page, and
> there is nothing else present in the page->compound_head. Doing an OR
> is more expensive than just setting it to 2.

Thanks for clarifying.

>
> I'm not really sure why set/clear page_pfmemalloc are defined in mm.h.
> They should probably be in mm/page_alloc.c where nobody else would ever
> think that they could or should be calling them.>
>>> struct { /* page_pool used by netstack */
>>> - /**
>>> - * @dma_addr: might require a 64-bit value on
>>> - * 32-bit architectures.
>>> - */
>>> + unsigned long pp_magic;
>>> + struct page_pool *pp;
>>> + unsigned long _pp_mapping_pad;
>>> unsigned long dma_addr[2];
>>
>> It seems the dma_addr[1] aliases with page->private, and
>> page_private() is used in skb_copy_ubufs()?
>>
>> It seems we can avoid using page_private() in skb_copy_ubufs()
>> by using a dynamic allocated array to store the page ptr?
>
> This is why I hate it when people use page_private() instead of
> documenting what they're doing in struct page. There is no way to know
> (as an outsider to networking) whether the page in skb_copy_ubufs()
> comes from page_pool. I looked at it, and thought it didn't:
>
> page = alloc_page(gfp_mask);
>
> but if you say those pages can come from page_pool, I believe you.

page_private() using in skb_copy_ubufs() does indeed seem ok here.
the page_private() is used on the page which is freshly allocated
from alloc_page().

Sorry for the confusion.

>
> .
>