Messages in this thread | | | Date | Fri, 28 Jul 2023 19:30:33 +0200 | From | David Hildenbrand <> | Subject | Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow_protnone fallout |
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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
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