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Subject: Re: [PATCH v2 net-next 1/2] tcp: add TCP_ZEROCOPY_RECEIVE support for
 zerocopy receive
To:     Eric Dumazet <edumazet@google.com>,
        "David S . Miller" <davem@davemloft.net>
Cc:     netdev <netdev@vger.kernel.org>, Andy Lutomirski <luto@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Soheil Hassas Yeganeh <soheil@google.com>
References: <20180425214307.159264-1-edumazet@google.com>
 <20180425214307.159264-2-edumazet@google.com>
From:   Ka-Cheong Poon <ka-cheong.poon@oracle.com>
Organization: Oracle Corporation
Message-ID: <d3ad6970-4139-76a9-2417-3df077753aa9@oracle.com>
Date:   Thu, 26 Apr 2018 21:40:14 +0800
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On 04/26/2018 05:43 AM, Eric Dumazet wrote:
> When adding tcp mmap() implementation, I forgot that socket lock
> had to be taken before current->mm->mmap_sem. syzbot eventually caught
> the bug.
> 
> Since we can not lock the socket in tcp mmap() handler we have to
> split the operation in two phases.
> 
> 1) mmap() on a tcp socket simply reserves VMA space, and nothing else.
>    This operation does not involve any TCP locking.
> 
> 2) setsockopt(fd, IPPROTO_TCP, TCP_ZEROCOPY_RECEIVE, ...) implements
>   the transfert of pages from skbs to one VMA.
>    This operation only uses down_read(&current->mm->mmap_sem) after
>    holding TCP lock, thus solving the lockdep issue.


A quick question.  Is it a normal practice to return a result
in setsockopt() given that the optval parameter is supposed to
be a const void *?


-- 
K. Poon
ka-cheong.poon@oracle.com