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

[David Hildenbrand]

On 27.07.23 22:30, Peter Xu wrote:
> On Thu, Jul 27, 2023 at 09:17:45PM +0200, David Hildenbrand wrote:
> > 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.
>
> Because the old code was written like that?

And it's not 2014 anymore. Nowadays we do have the right helper in place.

[...]

> > 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.
>
> It doesn't really violate with the name, IMHO - protnone can be either numa
> hint or PROT_NONE for real. As long as we return NULL for a FOLL_NUMA
> request we're achieving the goal we want - we guarantee a NUMA balancing to
> trigger with when FOLL_NUMA provided.  It doesn't need to guarantee
> anything else, afaiu.  The final check relies in vma_is_accessible() in the
> fault paths anyway.  So I don't blame the old name that much.

IMHO, the name FOLL_NUMA made sense when it still was called pte_numa.

> > 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.
>
> As stated above, to me FOLL_NUMA is all fine and clear.  If you think
> having a flag back for protnone is worthwhile no matter as-is (FOLL_NUMA)
> or with reverted meaning, then that sounds all fine to me. Maybe the old
> name at least makes old developers know what's that.
>
> I don't have a strong opinion on names though; mostly never had.

I'll avoid new FOLL_ flags first and post my proposal. If many people 
are unhappy with that approach, we can revert the commit and call it a day.

Thanks!

--
Cheers,

David / dhildenb