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;
>> }
>
>
> .
>
