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

[David Hildenbrand <david@redhat.com>]

On 16.01.23 14:47, Kirill A. Shutemov wrote:
> On Mon, Jan 16, 2023 at 02:07:41PM +0100, David Hildenbrand wrote:
>> On 16.01.23 13:34, Kirill A. Shutemov wrote:
>>> 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@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?
>>>
>>> Looks good to me.
>>>
>>>>> 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).
>>>
>>> I think such architectures must provide proper pmdp_collapse_flush()
>>> with the required serialization. Power and S390 already do that.
>>>
>>
>> The plan is to eventually move away from (ab)using IPI to synchronize with
>> GUP-fast. Moving further into that direction a is wrong.
>>
>> The flush was added as a quick fix for all architectures by Jann, until
>> we can do better.
>>
>> Even for ppc64, see:
>>
>> commit bedf03416913d88c796288f9dca109a53608c745
>> Author: Yang Shi <shy828301@gmail.com>
>> Date:   Wed Sep 7 11:01:44 2022 -0700
>>
>>      powerpc/64s/radix: don't need to broadcast IPI for radix pmd collapse flush
>>      The IPI broadcast is used to serialize against fast-GUP, but fast-GUP will
>>      move to use RCU instead of disabling local interrupts in fast-GUP.  Using
>>      an IPI is the old-styled way of serializing against fast-GUP although it
>>      still works as expected now.
>>      And fast-GUP now fixed the potential race with THP collapse by checking
>>      whether PMD is changed or not.  So IPI broadcast in radix pmd collapse
>>      flush is not necessary anymore.  But it is still needed for hash TLB.
> 
> Okay. But I think tlb_remove_table_sync_one() belongs inside
> pmdp_collapse_flush(). Collapsing pmd table into huge page without
> serialization is a bug. They should not be separate.

Agreed. But I wonder if it should be moved into a generic 
pmdp_collapse_flush(), that calls an arch specific __pmdp_collapse_flush().

-- 
Thanks,

David / dhildenb