Re: [PATCH v3 3/4] mm: don't expose non-hugetlb page to fast gup prematurely

[Yu Zhao]

On Tue, Oct 01, 2019 at 03:31:51PM -0700, John Hubbard wrote:
> On 9/26/19 10:06 PM, Yu Zhao wrote:
> > On Thu, Sep 26, 2019 at 08:26:46PM -0700, John Hubbard wrote:
> >> On 9/26/19 3:20 AM, Kirill A. Shutemov wrote:
> >>> On Wed, Sep 25, 2019 at 04:26:54PM -0600, Yu Zhao wrote:
> >>>> On Wed, Sep 25, 2019 at 10:25:30AM +0200, Peter Zijlstra wrote:
> >>>>> On Tue, Sep 24, 2019 at 05:24:58PM -0600, Yu Zhao wrote:
> >> ...
> >>>>> I'm thinking this patch make stuff rather fragile.. Should we instead
> >>>>> stick the barrier in set_p*d_at() instead? Or rather, make that store a
> >>>>> store-release?
> >>>>
> >>>> I prefer it this way too, but I suspected the majority would be
> >>>> concerned with the performance implications, especially those
> >>>> looping set_pte_at()s in mm/huge_memory.c.
> >>>
> >>> We can rename current set_pte_at() to __set_pte_at() or something and
> >>> leave it in places where barrier is not needed. The new set_pte_at()( will
> >>> be used in the rest of the places with the barrier inside.
> >>
> >> +1, sounds nice. I was unhappy about the wide-ranging changes that would have
> >> to be maintained. So this seems much better.
> > 
> > Just to be clear that doing so will add unnecessary barriers to one
> > of the two paths that share set_pte_at().
> 
> Good point, maybe there's a better place to do it...
> 
> 
> > 
> >>> BTW, have you looked at other levels of page table hierarchy. Do we have
> >>> the same issue for PMD/PUD/... pages?
> >>>
> >>
> >> Along the lines of "what other memory barriers might be missing for
> >> get_user_pages_fast(), I'm also concerned that the synchronization between
> >> get_user_pages_fast() and freeing the page tables might be technically broken,
> >> due to missing memory barriers on the get_user_pages_fast() side. Details:
> >>
> >> gup_fast() disables interrupts, but I think it also needs some sort of
> >> memory barrier(s), in order to prevent reads of the page table (gup_pgd_range,
> >> etc) from speculatively happening before the interrupts are disabled. 
> > 
> > I was under impression switching back from interrupt context is a
> > full barrier (otherwise wouldn't we be vulnerable to some side
> > channel attacks?), so the reader side wouldn't need explicit rmb.
> > 
> 
> Documentation/memory-barriers.txt points out:
> 
> INTERRUPT DISABLING FUNCTIONS
> -----------------------------
> 
> Functions that disable interrupts (ACQUIRE equivalent) and enable interrupts
> (RELEASE equivalent) will act as compiler barriers only.  So if memory or I/O
> barriers are required in such a situation, they must be provided from some
> other means.
> 
> btw, I'm really sorry I missed your responses over the last 3 or 4 days.
> I just tracked down something in our email system that was sometimes
> moving some emails to spam (just few enough to escape immediate attention, argghh!).
> I think I killed it off for good now. I wasn't ignoring you. :)

Thanks, John. I agree with all you said, including the irq disabling
function not being a sufficient smp_rmb().

I was hoping somebody could clarify whether ipi handlers used by tlb
flush are sufficient to prevent CPU 1 from seeing any stale data from
freed page tables on all supported archs.

	CPU 1			CPU 2

				flush remote tlb by ipi
				wait for the ipi hanlder
	<ipi handler>
				free page table
	disable irq
	walk page table
	enable irq

I think they should because otherwise tlb flush wouldn't work if CPU 1
still sees stale data from the freed page table, unless there is a
really strange CPU cache design I'm not aware of.

Quoting comments from x86 ipi handler flush_tlb_func_common():
 * read active_mm's tlb_gen.  We don't need any explicit barriers
 * because all x86 flush operations are serializing and the
 * atomic64_read operation won't be reordered by the compiler.

For ppc64 ipi hander radix__flush_tlb_range(), there is an "eieio"
instruction:
  https://www.ibm.com/support/knowledgecenter/en/ssw_aix_72/assembler/idalangref_eieio_instrs.html
I'm not sure why it's not "sync" -- I'd guess something implicitly
works as "sync" already (or it's a bug).

I didn't find an ipi handler for tlb flush on arm64. There should be
one, otherwise fast gup on arm64 would be broken. Mark?