Re: [PATCH net-next v9 03/13] mm: page_frag: use initial zero offset for page_frag_alloc_align()

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2024/7/2 7:27, Alexander H Duyck wrote:
> On Tue, 2024-06-25 at 21:52 +0800, Yunsheng Lin wrote:
>> We are above to use page_frag_alloc_*() API to not just
> "about to use", not "above to use"

Ack.

> 
>> allocate memory for skb->data, but also use them to do
>> the memory allocation for skb frag too. Currently the
>> implementation of page_frag in mm subsystem is running
>> the offset as a countdown rather than count-up value,
>> there may have several advantages to that as mentioned
>> in [1], but it may have some disadvantages, for example,
>> it may disable skb frag coaleasing and more correct cache
>> prefetching
>>

...

>> diff --git a/mm/page_frag_cache.c b/mm/page_frag_cache.c
>> index 88f567ef0e29..da244851b8a4 100644
>> --- a/mm/page_frag_cache.c
>> +++ b/mm/page_frag_cache.c
>> @@ -72,10 +72,6 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
>>  		if (!page)
>>  			return NULL;
>>  
>> -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
>> -		/* if size can vary use size else just use PAGE_SIZE */
>> -		size = nc->size;
>> -#endif
>>  		/* Even if we own the page, we do not use atomic_set().
>>  		 * This would break get_page_unless_zero() users.
>>  		 */
>> @@ -84,11 +80,16 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
>>  		/* reset page count bias and offset to start of new frag */
>>  		nc->pfmemalloc = page_is_pfmemalloc(page);
>>  		nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
>> -		nc->offset = size;
>> +		nc->offset = 0;
>>  	}
>>  
>> -	offset = nc->offset - fragsz;
>> -	if (unlikely(offset < 0)) {
>> +#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
>> +	/* if size can vary use size else just use PAGE_SIZE */
>> +	size = nc->size;
>> +#endif
>> +
>> +	offset = __ALIGN_KERNEL_MASK(nc->offset, ~align_mask);
>> +	if (unlikely(offset + fragsz > size)) {
> 
> The fragsz check below could be moved to here.
> 
>>  		page = virt_to_page(nc->va);
>>  
>>  		if (!page_ref_sub_and_test(page, nc->pagecnt_bias))
>> @@ -99,17 +100,13 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
>>  			goto refill;
>>  		}
>>  
>> -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
>> -		/* if size can vary use size else just use PAGE_SIZE */
>> -		size = nc->size;
>> -#endif
>>  		/* OK, page count is 0, we can safely set it */
>>  		set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
>>  
>>  		/* reset page count bias and offset to start of new frag */
>>  		nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
>> -		offset = size - fragsz;
>> -		if (unlikely(offset < 0)) {
>> +		offset = 0;
>> +		if (unlikely(fragsz > PAGE_SIZE)) {
> 
> Since we aren't taking advantage of the flag that is left after the
> subtraction we might just want to look at moving this piece up to just
> after the offset + fragsz check. That should prevent us from trying to
> refill if we have a request that is larger than a single page. In
> addition we could probably just drop the 3 PAGE_SIZE checks above as
> they would be redundant.

I am not sure I understand the 'drop the 3 PAGE_SIZE checks' part and
the 'redundant' part, where is the '3 PAGE_SIZE checks'? And why they
are redundant?

> 
>>  			/*
>>  			 * The caller is trying to allocate a fragment
>>  			 * with fragsz > PAGE_SIZE but the cache isn't big
>> @@ -124,8 +121,7 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
>>  	}
>>  
>>  	nc->pagecnt_bias--;
>> -	offset &= align_mask;
>> -	nc->offset = offset;
>> +	nc->offset = offset + fragsz;
>>  
>>  	return nc->va + offset;
>>  }
> 
> 
> .
>