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

[Christoph Hellwig]

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.