MSG_ZEROCOPY and the O_DIRECT vs fork() race

[David Howells <dhowells@redhat.com>]

Andrew Lunn <andrew@lunn.ch> wrote:

> > I'm looking into making the sendmsg() code properly handle the 'DIO vs
> > fork' issue (where pages need pinning rather than refs taken) and also
> > getting rid of the taking of refs entirely as the page refcount is going
> > to go away in the relatively near future.
> 
> Sorry, new to this conversation, and i don't know what you mean by DIO
> vs fork.

As I understand it, there's a race between O_DIRECT I/O and fork whereby if
you, say, start a DIO read operation on a page and then fork, the target page
gets attached to child and a copy made for the parent (because the refcount is
elevated by the I/O) - and so only the child sees the result.  This is made
more interesting by such as AIO where the parent gets the completion
notification, but not the data.

Further, a DIO write is then alterable by the child if the DMA has not yet
happened.

One of the things mm/gup.c does is to work around this issue...  However, I
don't think that MSG_ZEROCOPY handles this - and so zerocopy sendmsg is, I
think, subject to the same race.

> Could you point me at a discussion.

I don't know of one, offhand, apart from in the logs for mm/gup.c.  I've added
a couple more mm guys and the mm list to the cc: field.

The information in the description of fc1d8e7cca2daa18d2fe56b94874848adf89d7f5
may be relevant.

David