Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow_protnone fallout

[David Hildenbrand <david@redhat.com>]

On 28.07.23 18:18, Linus Torvalds wrote:
> On Thu, 27 Jul 2023 at 14:28, David Hildenbrand <david@redhat.com> wrote:
>>
>> This is my proposal on how to handle the fallout of 474098edac26
>> ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()") where I
>> accidentially missed that follow_page() and smaps implicitly kept the
>> FOLL_NUMA flag clear by *not* setting it if FOLL_FORCE is absent, to
>> not trigger faults on PROT_NONE-mapped PTEs.
> 
> Ugh.

I was hoping for that reaction, with the assumption that we would get
something cleaner :)

> 
> I hate how it uses FOLL_FORCE that is inherently scary.

I hate FOLL_FORCE, but I hate FOLL_NUMA even more, because to me it
is FOLL_FORCE in disguise (currently and before 474098edac26, if
FOLL_FORCE is set, FOLL_NUMA won't be set and the other way around).

> 
> Why do we have that "gup_can_follow_protnone()" logic AT ALL?

That's what I was hoping for.

> 
> Couldn't we just get rid of that disgusting thing, and just say that
> GUP (and follow_page()) always just ignores NUMA hinting, and always
> just follows protnone?
> 
> We literally used to have this:
> 
>          if (!(gup_flags & FOLL_FORCE))
>                  gup_flags |= FOLL_NUMA;
> 
> ie we *always* set FOLL_NUMA for any sane situation. FOLL_FORCE should
> be the rare crazy case.

Yes, but my point would be that we now spell that "rare crazy case"
out for follow_page().

If you're talking about patch #1, I agree, therefore patch #3 to
avoid all that nasty FOLL_FORCE handling in GUP callers.

But yeah, if we can avoid all that, great.

> 
> The original reason for not setting FOLL_NUMA all the time is
> documented in commit 0b9d705297b2 ("mm: numa: Support NUMA hinting
> page faults from gup/gup_fast") from way back in 2012:
> 
>           * If FOLL_FORCE and FOLL_NUMA are both set, handle_mm_fault
>           * would be called on PROT_NONE ranges. We must never invoke
>           * handle_mm_fault on PROT_NONE ranges or the NUMA hinting
>           * page faults would unprotect the PROT_NONE ranges if
>           * _PAGE_NUMA and _PAGE_PROTNONE are sharing the same pte/pmd
>           * bitflag. So to avoid that, don't set FOLL_NUMA if
>           * FOLL_FORCE is set.


In handle_mm_fault(), we never call do_numa_page() if
!vma_is_accessible(). Same for do_huge_pmd_numa_page().

So, if we would ever end up triggering a page fault on
mprotect(PROT_NONE) ranges (i.e., via FOLL_FORCE), we
would simply do nothing.

At least that's the hope, I'll take a closer look just to make
sure we're good on all call paths.

> 
> but I don't think the original reason for this is *true* any more.
> 
> Because then two years later in 2014, in commit c46a7c817e66 ("x86:
> define _PAGE_NUMA by reusing software bits on the PMD and PTE levels")
> Mel made the code able to distinguish between PROT_NONE and NUMA
> pages, and he changed the comment above too.

CCing Mel.

I remember that pte_protnone() can only distinguished between
NUMA vs. actual mprotect(PROT_NONE) by looking at the VMA -- vma_is_accessible().

Indeed, include/linux/pgtable.h:

/*
  * Technically a PTE can be PROTNONE even when not doing NUMA balancing but
  * the only case the kernel cares is for NUMA balancing and is only ever set
  * when the VMA is accessible. For PROT_NONE VMAs, the PTEs are not marked
  * _PAGE_PROTNONE so by default, implement the helper as "always no". It
  * is the responsibility of the caller to distinguish between PROT_NONE
  * protections and NUMA hinting fault protections.
  */

> 
> But I get the very very strong feeling that instead of changing the
> comment, he should have actually removed the comment and the code.
> 
> So I get the strong feeling that all these FOLL_NUMA games should just
> go away. You removed the FOLL_NUMA bit, but you replaced it with using
> FOLL_FORCE.
> 
> So rather than make this all even more opaque and make it even harder
> to figure out why we have that code in the first place, I think it
> should all just be removed.

At least to me, spelling FOLL_FORCE in follow_page() now out is much
less opaque then getting that implied by lack of FOLL_NUMA.

> 
> The original reason for FOLL_NUMA simply does not exist any more. We
> know exactly when a page is marked for NUMA faulting, and we should
> simply *ignore* it for GUP and follow_page().
> 
> I think we should treat a NUMA-faulting page as just being present
> (and not NUMA-fault it).
> 
> Am I missing something?

There was the case for "FOLL_PIN represents application behavior and
should trigger NUMA faults", but I guess that can be ignored.

But it would be much better to just remove all that if we can.

Let me look into some details.

Thanks Linus!

-- 
Cheers,

David / dhildenb