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

[Matthew Wilcox <willy@infradead.org>]

On Wed, Apr 25, 2018 at 06:01:02AM -0700, Eric Dumazet wrote:
> On 04/24/2018 11:28 PM, Christoph Hellwig wrote:
> > 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.
> > 
> 
> Thanks Christoph
> 
> Note the high cost of zap_page_range(), needed to avoid -EBUSY being returned
> from vm_insert_page() the second time TCP_ZEROCOPY_RECEIVE is used on one VMA.
> 
> Ideally a vm_replace_page() would avoid this cost ?

If you don't zap the page range, any of the CPUs in the system where
any thread in this task have ever run may have a TLB entry pointing to
this page ... if the page is being recycled into the page allocator,
then that page might end up as a slab page or page table or page cache
while the other CPU still have access to it.

You could hang onto the page until you've built up a sufficiently large
batch, then bulk-invalidate all of the TLB entries, but we start to get
into weirdnesses on different CPU architectures.