linux-kernel - Re: [RFC PATCH v3 07/12] page-pool: device memory support

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Message-ID: <CAHS8izNbw7vAGo2euQGA+TF9CgQ8zwrDqTVGsOSxh22_uo0R1w@mail.gmail.com>
Date:   Tue, 7 Nov 2023 13:56:51 -0800
From:   Mina Almasry <almasrymina@...gle.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Ahern <dsahern@...nel.org>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Shuah Khan <shuah@...nel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Jeroen de Borst <jeroendb@...gle.com>,
        Praveen Kaligineedi <pkaligineedi@...gle.com>
Subject: Re: [RFC PATCH v3 07/12] page-pool: device memory support

On Tue, Nov 7, 2023 at 12:00 AM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>
> On 2023/11/6 10:44, Mina Almasry wrote:
> > Overload the LSB of struct page* to indicate that it's a page_pool_iov.
> >
> > Refactor mm calls on struct page* into helpers, and add page_pool_iov
> > handling on those helpers. Modify callers of these mm APIs with calls to
> > these helpers instead.
> >
> > In areas where struct page* is dereferenced, add a check for special
> > handling of page_pool_iov.
> >
> > Signed-off-by: Mina Almasry <almasrymina@...gle.com>
> >
> > ---
> >  include/net/page_pool/helpers.h | 74 ++++++++++++++++++++++++++++++++-
> >  net/core/page_pool.c            | 63 ++++++++++++++++++++--------
> >  2 files changed, 118 insertions(+), 19 deletions(-)
> >
> > diff --git a/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h
> > index b93243c2a640..08f1a2cc70d2 100644
> > --- a/include/net/page_pool/helpers.h
> > +++ b/include/net/page_pool/helpers.h
> > @@ -151,6 +151,64 @@ static inline struct page_pool_iov *page_to_page_pool_iov(struct page *page)
> >       return NULL;
> >  }
> >
> > +static inline int page_pool_page_ref_count(struct page *page)
> > +{
> > +     if (page_is_page_pool_iov(page))
> > +             return page_pool_iov_refcount(page_to_page_pool_iov(page));
>
> We have added a lot of 'if' for the devmem case, it would be better to
> make it more generic so that we can have more unified metadata handling
> for normal page and devmem. If we add another memory type here, do we
> need another 'if' here?

Maybe, not sure. I'm guessing new memory types will either be pages or
iovs, so maybe no new if statements needed.

> That is part of the reason I suggested using a more unified metadata for
> all the types of memory chunks used by page_pool.

I think your suggestion was to use struct pages for devmem. That was
thoroughly considered and intensely argued about in the initial
conversations regarding devmem and the initial RFC, and from the
conclusions there it's extremely clear to me that devmem struct pages
are categorically a no-go.

--
Thanks,
Mina