Hi Yunsheng,
[...]
> + * 1. 'n == 1': no need to actually overwrite it.
> + * 2. 'n != 1': overwrite it with one, which is the rare case
> + * for pp_frag_count draining.
> *
> - * The main advantage to doing this is that an atomic_read is
> - * generally a much cheaper operation than an atomic update,
> - * especially when dealing with a page that may be partitioned
> - * into only 2 or 3 pieces.
> + * The main advantage to doing this is that not only we avoid a atomic
> + * update, as an atomic_read is generally a much cheaper operation than
> + * an atomic update, especially when dealing with a page that may be
> + * partitioned into only 2 or 3 pieces; but also unify the pp_frag_count
> + * handling by ensuring all pages have partitioned into only 1 piece
> + * initially, and only overwrite it when the page is partitioned into
> + * more than one piece.
> */
> - if (atomic_long_read(&page->pp_frag_count) == nr)
> + if (atomic_long_read(&page->pp_frag_count) == nr) {
> + /* As we have ensured nr is always one for constant case using
> + * the BUILD_BUG_ON(), only need to handle the non-constant case
> + * here for pp_frag_count draining, which is a rare case.
> + */
> + BUILD_BUG_ON(__builtin_constant_p(nr) && nr != 1);
> + if (!__builtin_constant_p(nr))
> + atomic_long_set(&page->pp_frag_count, 1);
Aren't we changing the behaviour of the current code here? IIRC is
atomic_long_read(&page->pp_frag_count) == nr we never updated the atomic
pp_frag_count and the reasoning was that the next caller can set it
properly.
> +
> return 0;
> + }
>
> ret = atomic_long_sub_return(nr, &page->pp_frag_count);
> WARN_ON(ret < 0);
> +
> + /* We are the last user here too, reset pp_frag_count back to 1 to
> + * ensure all pages have been partitioned into 1 piece initially,
> + * this should be the rare case when the last two fragment users call
> + * page_pool_defrag_page() currently.
> + */
> + if (unlikely(!ret))
> + atomic_long_set(&page->pp_frag_count, 1);
> +
> return ret;
> }
>
[....]
Thanks
/Ilias