linux-kernel - Re: [PATCH net-next v12 1/5] page_pool: unify frag_count handling in page_pool_is_last

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Date:   Tue, 24 Oct 2023 11:26:56 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Liang Chen <liangchen.linux@...il.com>,
        Alexander Lobakin <aleksander.lobakin@...el.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next v12 1/5] page_pool: unify frag_count handling in page_pool_is_last_frag()

On Mon, 23 Oct 2023 at 15:27, Yunsheng Lin <linyunsheng@...wei.com> wrote:
>
> On 2023/10/23 19:43, Ilias Apalodimas wrote:
> > 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.
>
> If the next caller is calling the page_pool_alloc_frag(), then yes,
> because page_pool_fragment_page() will be used to reset the
> page->pp_frag_count, so it does not really matter what is the value
> of page->pp_frag_count when we are recycling a page.
>
> If the next caller is calling page_pool_alloc_pages() directly without
> fragmenting a page, the above code is used to ensure that pp_frag_count
> is always one when page_pool_alloc_pages() fetches a page from pool->alloc
> or pool->ring, because page_pool_fragment_page() is not used to reset the
> page->pp_frag_count for page_pool_alloc_pages() and we have removed the
> per page_pool PP_FLAG_PAGE_FRAG in page_pool_is_last_frag().
>
> As we don't know if the caller is page_pool_alloc_frag() or
> page_pool_alloc_pages(), so the above code ensure the page in pool->alloc
> or pool->ring always have the pp_frag_count being one.


Fair enough, Jakub pulled the series before I managed to ack them, but
that's okay. It's been long overdue apologies. FWIW I went through the
patches and didn't find anything wrong coding-wise

Thanks
/Ilias
>
>
>
> >
> >> +
> >>              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
> >
> > .
> >