Re: [PATCH] smaps: Fix the abnormal memory statistics obtained through /proc/pid/smaps

[David Hildenbrand]

On 27.07.23 20:59, Peter Xu wrote:
> On Thu, Jul 27, 2023 at 07:27:02PM +0200, David Hildenbrand wrote:
> > > > This was wrong from the very start. If we're not in GUP, we shouldn't call
> > > > GUP functions.
> > >
> > > My understanding is !GET && !PIN is also called gup.. otherwise we don't
> > > need GET and it can just be always implied.
> >
> > That's not the point. The point is that _arbitrary_ code shouldn't call into
> > GUP internal helper functions, where they bypass, for example, any sanity
> > checks.
>
> What's the sanity checks that you're referring to?

For example in follow_page()

if (vma_is_secretmem(vma))
	return NULL;

if (WARN_ON_ONCE(foll_flags & FOLL_PIN))
	return NULL;


Maybe you can elaborate why you think we should *not* be using 
vm_normal_page_pmd() and instead some arbitrary GUP internal helper? I 
don't get it.

> > > The other proof is try_grab_page() doesn't fail hard on !GET && !PIN.  So I
> > > don't know whether that's "wrong" to be used..
> >
> > To me, that is arbitrary code using a GUP internal helper and, therefore,
> > wrong.
> >
> > > Back to the topic: I'd say either of the patches look good to solve the
> > > problem.  If p2pdma pages are mapped as PFNMAP/MIXEDMAP (?), I guess
> > > vm_normal_page_pmd() proposed here will also work on it, so nothing I see
> > > wrong on 2nd one yet.
> > >
> > > It looks nicer indeed to not have FOLL_FORCE here, but it also makes me
> > > just wonder whether we should document NUMA behavior for FOLL_* somewhere,
> > > because we have an implication right now on !FOLL_FORCE over NUMA, which is
> > > not obvious to me..
> >
> > Yes, we probably should. For get_use_pages() and friends that behavior was
> > always like that and it makes sense: usually it represent application
> > behavior.
> >
> > > And to look more over that aspect, see follow_page(): previously we can
> > > follow a page for protnone (as it never applies FOLL_NUMA) but now it won't
> > > (it never applies FOLL_FORCE, either, so it seems "accidentally" implies
> > > FOLL_NUMA now).  Not sure whether it's intended, though..
> >
> > That was certainly an oversight, thanks for spotting that. That patch was
> > not supposed to change semantics:
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 76d222ccc3ff..ac926e19ff72 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -851,6 +851,13 @@ struct page *follow_page(struct vm_area_struct *vma,
> > unsigned long address,
> >          if (WARN_ON_ONCE(foll_flags & FOLL_PIN))
> >                  return NULL;
> >
> > +       /*
> > +        * In contrast to get_user_pages() and friends, we don't want to
> > +        * fail if the PTE is PROT_NONE: see gup_can_follow_protnone().
> > +        */
> > +       if (!(foll_flags & FOLL_WRITE))
> > +               foll_flags |= FOLL_FORCE;
> > +
> >          page = follow_page_mask(vma, address, foll_flags, &ctx);
> >          if (ctx.pgmap)
> >                  put_dev_pagemap(ctx.pgmap);
>
> This seems to be slightly against your other solution though for smaps,
> where we want to avoid abusing FOLL_FORCE.. isn't it..

This is GUP internal, not some arbitrary code, so to me a *completely* 
different discussion.

> Why read only?  That'll always attach FOLL_FORCE to all follow page call
> sites indeed for now, but just curious - logically "I want to fetch the
> page even if protnone" is orthogonal to do with write permission here to
> me.

Historical these were not the semantics, so I won't change them.

FOLL_FORCE | FOLL_WRITE always had a special taste to it (COW ...).

> I still worry about further abuse of FOLL_FORCE, I believe you also worry
> that so you proposed the other way for the smaps issue.
>
> Do you think we can just revive FOLL_NUMA?  That'll be very clear to me
> from that aspect that we do still have valid use cases for it.

FOLL_NUMA naming was nowadays wrong to begin with (not to mention, 
confusing a we learned). There are other reasons why we have PROT_NONE 
-- mprotect(), for example.

We could have a flag that goes the other way around: 
FOLL_IGNORE_PROTNONE ... which surprisingly then ends up being exactly 
what FOLL_FORCE means without FOLL_WRITE, and what this patch does.

Does that make sense to you?


> The very least is if with above we should really document FOLL_FORCE - we
> should mention NUMA effects.  But that's ... really confusing. Thinking
> about that I personally prefer a revival of FOLL_NUMA, then smaps issue all
> go away.

smaps needs to be changed in any case IMHO. And I'm absolutely not in 
favor of revicing FOLL_NUMA.

--
Cheers,

David / dhildenb