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Subject: Re: [PATCH net-next v4 1/4] mm: add a signature in struct page
To:     Matthew Wilcox <willy@infradead.org>
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From:   Yunsheng Lin <linyunsheng@huawei.com>
Message-ID: <8f815871-e384-3e65-56a8-39e379dea4ce@huawei.com>
Date:   Thu, 13 May 2021 11:25:22 +0800
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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.

> 
> .
>