Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM

[John Hubbard]

On 10/3/20 2:45 AM, Daniel Vetter wrote:
> On Sat, Oct 3, 2020 at 12:39 AM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
> > On 10/2/20 10:53 AM, Daniel Vetter wrote:
> > > For $reasons I've stumbled over this code and I'm not sure the change
> > > to the new gup functions in 55a650c35fea ("mm/gup: frame_vector:
> > > convert get_user_pages() --> pin_user_pages()") was entirely correct.
> > >
> > > This here is used for long term buffers (not just quick I/O) like
> > > RDMA, and John notes this in his patch. But I thought the rule for
> > > these is that they need to add FOLL_LONGTERM, which John's patch
> > > didn't do.
> >
> > Yep. The earlier gup --> pup conversion patches were intended to not
> > have any noticeable behavior changes, and FOLL_LONGTERM, with it's
> > special cases and such, added some risk that I wasn't ready to take
> > on yet. Also, FOLL_LONGTERM rules are only *recently* getting firmed
> > up. So there was some doubt at least in my mind, about which sites
> > should have it.
> >
> > But now that we're here, I think it's really good that you've brought
> > this up. It's definitely time to add FOLL_LONGTERM wherever it's missing.
>
> So should I keep this patch, or will it collide with a series you're working on?

It doesn't collide with anything on my end yet, because I've been slow to
pick up on the need for changing callsites to add FOLL_LONGTERM. :)

And it looks like that's actually a problem, because:

> Also with the firmed up rules, correct that I can also drop the
> vma_is_fsdax check when the FOLL_LONGTERM flag is set?

That's the right direction to go *in general*, but I see that the
pin_user_pages code is still a bit stuck in the past. And this patch
won't actually work, with or without that vma_is_fsdax() check.
Because:

get_vaddr_frames(FOLL_LONGTERM)
   pin_user_pages_locked()
	if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
		return -EINVAL;


So, again, pin_user_pages*() is at least partly behind the times here.
I can jump in and start fixing it up, but it depends on what you and
Oded and others are planning? Note: there is a particular combination of
dax and locking that we have to still avoid, within gup.c. That's
already covered, but needs to continue to be covered when we enable
FOLL_LONGTERM in the remaining pin_user_pages*() calling paths.



thanks,
--
John Hubbard
NVIDIA