Re: [PATCH 1/3] mm/mlock.c: convert put_page() to put_user_page*()

[John Hubbard]

On 8/9/19 1:23 AM, Michal Hocko wrote:
> On Fri 09-08-19 10:12:48, Vlastimil Babka wrote:
> > On 8/9/19 12:59 AM, John Hubbard wrote:
> > > > > That's true. However, I'm not sure munlocking is where the
> > > > > put_user_page() machinery is intended to be used anyway? These are
> > > > > short-term pins for struct page manipulation, not e.g. dirtying of page
> > > > > contents. Reading commit fc1d8e7cca2d I don't think this case falls
> > > > > within the reasoning there. Perhaps not all GUP users should be
> > > > > converted to the planned separate GUP tracking, and instead we should
> > > > > have a GUP/follow_page_mask() variant that keeps using get_page/put_page?
> > > > >   
> > > >
> > > > Interesting. So far, the approach has been to get all the gup callers to
> > > > release via put_user_page(), but if we add in Jan's and Ira's vaddr_pin_pages()
> > > > wrapper, then maybe we could leave some sites unconverted.
> > > >
> > > > However, in order to do so, we would have to change things so that we have
> > > > one set of APIs (gup) that do *not* increment a pin count, and another set
> > > > (vaddr_pin_pages) that do.
> > > >
> > > > Is that where we want to go...?
> >
> > We already have a FOLL_LONGTERM flag, isn't that somehow related? And if
> > it's not exactly the same thing, perhaps a new gup flag to distinguish
> > which kind of pinning to use?
>
> Agreed. This is a shiny example how forcing all existing gup users into
> the new scheme is subotimal at best. Not the mention the overal
> fragility mention elsewhere. I dislike the conversion even more now.
>
> Sorry if this was already discussed already but why the new pinning is
> not bound to FOLL_LONGTERM (ideally hidden by an interface so that users
> do not have to care about the flag) only?

Oh, it's been discussed alright, but given how some of the discussions have gone,
I certainly am not surprised that there are still questions and criticisms!
Especially since I may have misunderstood some of the points, along the way.
It's been quite a merry go round. :)

Anyway, what I'm hearing now is: for gup(FOLL_LONGTERM), apply the pinned tracking.
And therefore only do put_user_page() on pages that were pinned with
FOLL_LONGTERM. For short term pins, let the locking do what it will:
things can briefly block and all will be well.

Also, that may or may not come with a wrapper function, courtesy of Jan
and Ira.

Is that about right? It's late here, but I don't immediately recall any
problems with doing it that way...

thanks,
--
John Hubbard
NVIDIA