Re: [PATCH] mm/khugepaged: Fix ->anon_vma race

[Kirill A. Shutemov]

On Mon, Jan 16, 2023 at 01:06:59PM +0100, Jann Horn wrote:
> On Sun, Jan 15, 2023 at 8:07 PM Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
> > On Fri, Jan 13, 2023 at 08:28:59PM +0100, Jann Horn wrote:
> > > No, that lockdep assert has to be there. Page table traversal is
> > > allowed under any one of the mmap lock, the anon_vma lock (if the VMA
> > > is associated with an anon_vma), and the mapping lock (if the VMA is
> > > associated with a mapping); and so to be able to remove page tables,
> > > we must hold all three of them.
> >
> > Okay, that's fair. I agree with the patch now. Maybe adjust the commit
> > message a bit?
> 
> Just to make sure we're on the same page: Are you suggesting that I
> add this text?
> "Page table traversal is allowed under any one of the mmap lock, the
> anon_vma lock (if the VMA is associated with an anon_vma), and the
> mapping lock (if the VMA is associated with a mapping); and so to be
> able to remove page tables, we must hold all three of them."
> Or something else?

Looks good to me.

> > Anyway:
> >
> > Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> 
> Thanks!
> 
> > BTW, I've noticied that you recently added tlb_remove_table_sync_one().
> > I'm not sure why it is needed. Why IPI in pmdp_collapse_flush() in not
> > good enough to serialize against GUP fast?
> 
> If that sent an IPI, it would be good enough; but
> pmdp_collapse_flush() is not guaranteed to send an IPI.
> It does a TLB flush, but on some architectures (including arm64 and
> also virtualized x86), a remote TLB flush can be done without an IPI.
> For example, arm64 has some fancy hardware support for remote TLB
> invalidation without IPIs ("broadcast TLB invalidation"), and
> virtualized x86 has (depending on the hypervisor) things like TLB
> shootdown hypercalls (under Hyper-V, see hyperv_flush_tlb_multi) or
> TLB shootdown signalling for preempted CPUs through shared memory
> (under KVM, see kvm_flush_tlb_multi).

I think such architectures must provide proper pmdp_collapse_flush()
with the required serialization. Power and S390 already do that.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov