Re: [PATCH/RFC G-U-P experts] IB/umem: Modernize our get_user_pages() parameters

[Roland Dreier <roland@kernel.org>]

> Sigh, what a mess ... it seems what we really want to do is know
> if userspace might trigger a COW because or not, and only do a
> preemptive COW in that case.  (We're not really concerned with
> userspace fork()ing and setting up a COW in the future, since that's
> what we have MADV_DONTFORK for)
>
> The status quo works for userspace anonymous mappings but
> it doesn't work for my case of mapping a kernel buffer read-only
> into userspace.  And fixing my case breaks the anonymous case.
> Do you see a way out of this dilemma?  Do we need to add yet
> another flag to get_user_pages()?

So thinking about this a bit more... it seems what we want is at least
to first order that we do the equivalent of write==1 exactly when the vma
for a mapping has VM_WRITE set (or is it VMA_MAYWRITE / force==1?
I don't quite understand the distinction between WRITE and MAYWRITE).

Right now, one call to get_user_pages() might involve more than one vma,
but we could simulate the above by doing find_vma() and making sure our
call to get_user_pages() goes one vma at a time.  Of course that would be
inefficient since get_user_pages() will redo the find_vma() internally, so it
would I guess make sense to add another FOLL_ flag to tell
get_user_pages() to do this?

Am I all wet, or am I becoming an MM hacker?

Thanks,
  Roland