[Linuxarm] Re: [PATCH net-next v2 2/4] page_pool: add interface to manipulate frag count in page pool

On 06/08/2021 04.46, Yunsheng Lin wrote:

+static inline long page_pool_atomic_sub_frag_count_return(struct page *page,

• 					  long nr)
  

+{

• long ret;
• /* As suggested by Alexander, atomic_long_read() may cover up the

• * reference count errors, so avoid calling atomic_long_read() in
  

• * the cases of freeing or draining the page_frags, where we would
  

• * not expect it to match or that are slowpath anyway.
  

• */
  

• if (__builtin_constant_p(nr) &&

•    atomic_long_read(&page->pp_frag_count) == nr)
  

• return 0;
  

• ret = atomic_long_sub_return(nr, &page->pp_frag_count);
• WARN_ON(ret < 0);

I worried about this WARN_ON() as it generates an 'ud2' instruction which influence I-cache fetching. But I have disassembled (objdump) the page_pool.o binary and the ud2 gets placed last in the main function page_pool_put_page() that use this inlined function. Thus, I assume this is not a problem :-)

• return ret;