Does GUP page unpinning have to be done in the pinning context?

[David Howells <dhowells@redhat.com>]

Hi Kirill, John, David,

I don't know if you're the experts on GUP, but can you tell me if unpinning,
e.g. unpin_user_page(), needs to be done in the same MM context and/or the
same user context as the initial pinning?  And, if so, is there some way to
break that link?

I'm looking at how I might extend page pinning into the socket layer and
sendmsg(MSG_ZEROCOPY).  The problem is that sendmsg() merely queues the
buffers with no guarantee that it's finished with them by the time it's
returned.  You get a SO_EE_ORIGIN_ZEROCOPY message to tell you that.

I have to deal with the issue that the process that did the sendmsg(), the MM
context and the user context may all have ceased to exist by the time the
transmission completes if I don't pin them.

Thanks,
David