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

[Jann Horn <jannh@google.com>]

On Sun, Jan 15, 2023 at 8:07 PM Kirill A. Shutemov <kirill@shutemov.name> 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?

> Anyway:
>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@intel.linux.com>

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).