Re: [PATCH net-next 1/2] page_pool: support non-split page with PP_FLAG_PAGE_FRAG

[Alexander Duyck]

On Mon, Aug 30, 2021 at 11:14 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>
> On 2021/8/30 23:05, Alexander Duyck wrote:
> > On Sun, Aug 29, 2021 at 6:19 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
> >>
> >> Currently when PP_FLAG_PAGE_FRAG is set, the caller is not
> >> expected to call page_pool_alloc_pages() directly because of
> >> the PP_FLAG_PAGE_FRAG checking in __page_pool_put_page().
> >>
> >> The patch removes the above checking to enable non-split page
> >> support when PP_FLAG_PAGE_FRAG is set.
> >>
> >> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
> >> ---
> >>  include/net/page_pool.h |  6 ++++++
> >>  net/core/page_pool.c    | 12 +++++++-----
> >>  2 files changed, 13 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> >> index a408240..2ad0706 100644
> >> --- a/include/net/page_pool.h
> >> +++ b/include/net/page_pool.h
> >> @@ -238,6 +238,9 @@ static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
> >>
> >>  static inline void page_pool_set_frag_count(struct page *page, long nr)
> >>  {
> >> +       if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
> >> +               return;
> >> +
> >>         atomic_long_set(&page->pp_frag_count, nr);
> >>  }
> >>
> >> @@ -246,6 +249,9 @@ static inline long page_pool_atomic_sub_frag_count_return(struct page *page,
> >>  {
> >>         long ret;
> >>
> >> +       if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
> >> +               return 0;
> >> +
> >>         /* 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
> >> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> >> index 1a69784..ba9f14d 100644
> >> --- a/net/core/page_pool.c
> >> +++ b/net/core/page_pool.c
> >> @@ -313,11 +313,14 @@ struct page *page_pool_alloc_pages(struct page_pool *pool, gfp_t gfp)
> >>
> >>         /* Fast-path: Get a page from cache */
> >>         page = __page_pool_get_cached(pool);
> >> -       if (page)
> >> -               return page;
> >>
> >>         /* Slow-path: cache empty, do real allocation */
> >> -       page = __page_pool_alloc_pages_slow(pool, gfp);
> >> +       if (!page)
> >> +               page = __page_pool_alloc_pages_slow(pool, gfp);
> >> +
> >> +       if (likely(page))
> >> +               page_pool_set_frag_count(page, 1);
> >> +
> >>         return page;
> >>  }
> >>  EXPORT_SYMBOL(page_pool_alloc_pages);
> >> @@ -426,8 +429,7 @@ __page_pool_put_page(struct page_pool *pool, struct page *page,
> >>                      unsigned int dma_sync_size, bool allow_direct)
> >>  {
> >>         /* It is not the last user for the page frag case */
> >> -       if (pool->p.flags & PP_FLAG_PAGE_FRAG &&
> >> -           page_pool_atomic_sub_frag_count_return(page, 1))
> >> +       if (page_pool_atomic_sub_frag_count_return(page, 1))
> >>                 return NULL;
> >
> > Isn't this going to have a negative performance impact on page pool
> > pages in general? Essentially you are adding an extra atomic operation
> > for all the non-frag pages.
> >
> > It would work better if this was doing a check against 1 to determine
> > if it is okay for this page to be freed here and only if the check
> > fails then you perform the atomic sub_return.
>
> The page_pool_atomic_sub_frag_count_return() has added the optimization
> to not do the atomic sub_return when the caller is the last user of the
> page, see page_pool_atomic_sub_frag_count_return():
>
>         /* 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;
>
> So the check against 1 is not needed here?

Ah, okay. I hadn't seen that part. So yeah, then this should be mostly
harmless since 1 falls into the category of a builtin constant and
would result in the standard case being the frag count being set to 1
and then being read which should be minimal overhead.

Reviewed-by: Alexander Duyck <alexanderduyck@xxxxxx>