Re: [PATCH rfc v6 2/4] page_pool: add interface to manipulate frag count in page pool

[Alexander Duyck <alexander.duyck@gmail.com>]

On Wed, Jul 21, 2021 at 1:15 AM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>
> On 2021/7/20 23:43, Alexander Duyck wrote:
> > On Mon, Jul 19, 2021 at 8:36 PM Yunsheng Lin <linyunsheng@huawei.com> wrote:
> >>
> >> For 32 bit systems with 64 bit dma, dma_addr[1] is used to
> >> store the upper 32 bit dma addr, those system should be rare
> >> those days.
> >>
> >> For normal system, the dma_addr[1] in 'struct page' is not
> >> used, so we can reuse dma_addr[1] for storing frag count,
> >> which means how many frags this page might be splited to.
> >>
> >> In order to simplify the page frag support in the page pool,
> >> the PAGE_POOL_DMA_USE_PP_FRAG_COUNT macro is added to indicate
> >> the 32 bit systems with 64 bit dma, and the page frag support
> >> in page pool is disabled for such system.
> >>
> >> The newly added page_pool_set_frag_count() is called to reserve
> >> the maximum frag count before any page frag is passed to the
> >> user. The page_pool_atomic_sub_frag_count_return() is called
> >> when user is done with the page frag.
> >>
> >> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
> >> ---
> >>  include/linux/mm_types.h | 18 +++++++++++++-----
> >>  include/net/page_pool.h  | 41 ++++++++++++++++++++++++++++++++++-------
> >>  net/core/page_pool.c     |  4 ++++
> >>  3 files changed, 51 insertions(+), 12 deletions(-)
> >>
> >
> > <snip>
> >
> >> +static inline long page_pool_atomic_sub_frag_count_return(struct page *page,
> >> +                                                         long nr)
> >> +{
> >> +       long frag_count = atomic_long_read(&page->pp_frag_count);
> >> +       long ret;
> >> +
> >> +       if (frag_count == nr)
> >> +               return 0;
> >> +
> >> +       ret = atomic_long_sub_return(nr, &page->pp_frag_count);
> >> +       WARN_ON(ret < 0);
> >> +       return ret;
> >>  }
> >>
> >
> > So this should just be an atomic_long_sub_return call. You should get
> > rid of the atomic_long_read portion of this as it can cover up
> > reference count errors.
>
> atomic_long_sub_return() is used to avoid one possible cache bouncing and
> barrrier caused by the last user.

I assume you mean "atomic_long_read()" here.

> You are right that that may cover up the reference count errors. How about
> something like below:
>
> static inline long page_pool_atomic_sub_frag_count_return(struct page *page,
>                                                           long nr)
> {
> #ifdef CONFIG_DEBUG_PAGE_REF
>         long ret = atomic_long_sub_return(nr, &page->pp_frag_count);
>
>         WARN_ON(ret < 0);
>
>         return ret;
> #else
>         if (atomic_long_read(&page->pp_frag_count) == nr)
>                 return 0;
>
>         return atomic_long_sub_return(nr, &page->pp_frag_count);
> #end
> }
>
> Or any better suggestion?

So the one thing I might change would be to make it so that you only
do the atomic_long_read if nr is a constant via __builtin_constant_p.
That way you would be performing the comparison in
__page_pool_put_page and in the cases of freeing or draining the
page_frags you would be using the atomic_long_sub_return which should
be paths where you would not expect it to match or that are slowpath
anyway.

Also I would keep the WARN_ON in both paths just to be on the safe side.