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Message-ID: <19902a48-c59b-4e3b-afc5-e792506c2fd6@redhat.com>
Date: Thu, 1 Aug 2024 15:13:26 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org,
willy@...radead.org
Cc: ryan.roberts@....com, anshuman.khandual@....com, catalin.marinas@....com,
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dave.hansen@...ux.intel.com, will@...nel.org, baohua@...nel.org,
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Subject: Re: Race condition observed between page migration and page fault
handling on arm64 machines
>>> To dampen the tradeoff, we could do this in shmem_fault() instead? But
>>> then, this would mean that we do this in all
>>>
>>> kinds of vma->vm_ops->fault, only when we discover another reference
>>> count race condition :) Doing this in do_fault()
>>>
>>> should solve this once and for all. In fact, do_pte_missing() may call
>>> do_anonymous_page() or do_fault(), and I just
>>>
>>> noticed that the former already checks this using vmf_pte_changed().
>>
>> What I am still missing is why this is (a) arm64 only; and (b) if this
>> is something we should really worry about. There are other reasons
>> (e.g., speculative references) why migration could temporarily fail,
>> does it happen that often that it is really something we have to worry
>> about?
>
>
> (a) See discussion at [1]; I guess it passes on x86, which is quite
> strange since the race is clearly arch-independent.
Yes, I think this is what we have to understand. Is the race simply less
likely to trigger on x86?
I would assume that it would trigger on any arch.
I just ran it on a x86 VM with 2 NUMA nodes and it also seems to work here.
Is this maybe related to deferred flushing? Such that the other CPU will
by accident just observe the !pte_none a little less likely?
But arm64 also usually defers flushes, right? At least unless
ARM64_WORKAROUND_REPEAT_TLBI is around. With that we never do deferred
flushes.
>
> (b) On my machine, on an average in under 10 iterations of move_pages(),
> it fails, which seems problematic to
Yes, it's a big difference compared to what I encounter.
--
Cheers,
David / dhildenb
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