netdev - Re: [PATCH net-next 1/2] tcp: add TCP_ZEROCOPY_RECEIVE support for zerocopy receive

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Date:   Tue, 24 Apr 2018 23:28:59 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next 1/2] tcp: add TCP_ZEROCOPY_RECEIVE support for
 zerocopy receive

On Tue, Apr 24, 2018 at 10:27:21PM -0700, 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.
> 
> This new implementation was suggested by Andy Lutomirski with great details.

Thanks, this looks much more sensible to me.