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Date: Tue, 23 Apr 2024 22:56:41 +0200
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Subject: Re: [PATCH v1 0/5] arm64/mm: uffd write-protect and soft-dirty
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To: Ryan Roberts <ryan.roberts@arm.com>, Mike Rapoport <rppt@linux.ibm.com>
Cc: Shivansh Vij <shivanshvij@outlook.com>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
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From: David Hildenbrand <david@redhat.com>
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>>>>
>>>> We further recently added a new UFFD_FEATURE_WP_ASYNC feature as part of
>>>> [2], because getting soft-dirty return reliable results in some cases turned
>>>> out rather hard to fix.
> 
> But it sounds like the current soft-dirty semantic is sufficient for CRIU on
> other arches? If I understood correctly from my brief scan of the linked post,
> the problem is that soft-dirty can sometimes provide false-positives? So could
> result in uneccessary copy, but never lost data?

Yes, it seems to be good enough for them in that regard I think.

[...]

>>>
>>>> But I'll throw in another idea: do we really need soft-dirty and uffd-wp to
>>>> exist at the same time in the same process (or the VMA?). In theory, we
> 
> My instinct is that MUXing a PTE bit like this will lead to some subtle problems
> that won't appear on arches that support either one or both of the features
> independently and unconditionally. Surely better to limit ourselves to either
> "arm64 will only support uffd-wp" or "arm64 will support both uffd-wp and
> soft-dirty". That way, we could move ahead with reviewing/merging the uffd-wp
> support asynchronously to deciding whether we want to support soft-dirty.

Yes. MUXing would require some work, but likely better than wasting 1/64 
PTE space on a corner case feature with one famous user that might be 
able to port to an alternative with other active users (growing ;) ).

Anyhow, I don't maintain arm64 code and we have to carry that baggage in 
the core either way for the time being ...

-- 
Cheers,

David / dhildenb