linux-kernel - Re: [PATCH v1 0/5] arm64/mm: uffd write-protect and soft-dirty tracking

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Message-ID: <8dec06c3-62d6-47c8-aafb-61c7f01f92b4@redhat.com>
Date: Tue, 23 Apr 2024 22:56:41 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, Mike Rapoport <rppt@...ux.ibm.com>
Cc: Shivansh Vij <shivanshvij@...look.com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>,
 Joey Gouly <joey.gouly@....com>, Ard Biesheuvel <ardb@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Anshuman Khandual <anshuman.khandual@....com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-arm-kernel@...ts.infradead.org"
 <linux-arm-kernel@...ts.infradead.org>,
 "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v1 0/5] arm64/mm: uffd write-protect and soft-dirty
 tracking


>>>>
>>>> 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