netdev - Re: [RFC PATCH 06/10] net: add SO_DEVMEM_DONTNEED setsockopt to release RX pages

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Date: Sun, 16 Jul 2023 19:06:44 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Andy Lutomirski <luto@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-media@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org, 
	netdev@...r.kernel.org, linux-arch@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, Sumit Semwal <sumit.semwal@...aro.org>, 
	Christian König <christian.koenig@....com>, 
	"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>, jgg@...pe.ca
Subject: Re: [RFC PATCH 06/10] net: add SO_DEVMEM_DONTNEED setsockopt to
 release RX pages

On Sun, Jul 16, 2023 at 4:57 PM Andy Lutomirski <luto@...nel.org> wrote:
>
> On 7/10/23 15:32, Mina Almasry wrote:
> > Add an interface for the user to notify the kernel that it is done reading
> > the NET_RX dmabuf pages returned as cmsg. The kernel will drop the
> > reference on the NET_RX pages to make them available for re-use.
> >
> > Signed-off-by: Mina Almasry <almasrymina@...gle.com>
> > ---
>
> > +             for (i = 0; i < num_tokens; i++) {
> > +                     for (j = 0; j < tokens[i].token_count; j++) {
> > +                             struct page *pg = xa_erase(&sk->sk_pagepool,
> > +                                                        tokens[i].token_start + j);
> > +
> > +                             if (pg)
> > +                                     put_page(pg);
> > +                             else
> > +                                     /* -EINTR here notifies the userspace
> > +                                      * that not all tokens passed to it have
> > +                                      * been freed.
> > +                                      */
> > +                                     ret = -EINTR;
>
> Unless I'm missing something, this type of error reporting is
> unrecoverable -- userspace doesn't know how many tokens have been freed.
>
> I think you should either make it explicitly unrecoverable (somehow shut
> down dmabuf handling entirely) or tell userspace how many tokens were
> successfully freed.
>

Thank you, the latter suggestion sounds perfect actually. The user
can't do much if the kernel fails to free all the tokens, but at least
it can know that something wrong is happening and can log an error for
further debugging. Great suggestion! Thanks!


-- 
Thanks,
Mina