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

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Date:   Sun, 16 Jul 2023 16:57:07 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Mina Almasry <almasrymina@...gle.com>,
        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
Cc:     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 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.

--Andy