Re: [PATCH net-next v21 13/14] mm: page_frag: update documentation for page_frag

[Yunsheng Lin]

On 2024/10/15 13:20, Bagas Sanjaya wrote:

...

>>  
>> +/**
>> + * page_frag_cache_is_pfmemalloc() - Check for pfmemalloc.
>> + * @nc: page_frag cache from which to check
>> + *
>> + * Used to check if the current page in page_frag cache is pfmemalloc'ed.
>                                                            is allocated by pfmemalloc()?

There seems to be no pfmemalloc() function.

Perhaps change it to something like below as the comment in
page_is_pfmemalloc():
Used to check if the current page in page_frag cache is allocated from the
pfmemalloc reserves.


>> + * It has the same calling context expectation as the alloc API.
>> + *
>> + * Return:
>> + * true if the current page in page_frag cache is pfmemalloc'ed, otherwise
>> + * return false.
>> + */
>>  static inline bool page_frag_cache_is_pfmemalloc(struct page_frag_cache *nc)
>>  {
>>  	return encoded_page_decode_pfmemalloc(nc->encoded_page);
>>  }
>>  
>> +/**
>> + * page_frag_cache_page_offset() - Return the current page fragment's offset.
>> + * @nc: page_frag cache from which to check
>> + *
>> + * The API is only used in net/sched/em_meta.c for historical reason, do not use
>> + * it for new caller unless there is a strong reason.
> 
> Then what does page_frag_cache_page_offset() do then?

It is used to replace the the below direct access of 'page_frag_cache':
https://elixir.bootlin.com/linux/v6.11/source/net/sched/em_meta.c#L585

Each one of 'page_frag_cache' instance has its own allocation context
to ensure lockless access, it is not encouraged to access 'page_frag_cache'
directly outside of the allocation context as the accessing is unreliable.