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To:     Lokesh Gidra <lokeshgidra@google.com>, Peter Xu <peterx@redhat.com>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI
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On 02.10.23 17:55, Lokesh Gidra wrote:
> On Mon, Oct 2, 2023 at 4:46 PM Lokesh Gidra <lokeshgidra@google.com> wrote:
>>
>> On Mon, Oct 2, 2023 at 4:21 PM Peter Xu <peterx@redhat.com> wrote:
>>>
>>> On Mon, Oct 02, 2023 at 10:00:03AM +0200, David Hildenbrand wrote:
>>>> In case we cannot simply remap the page, the fallback sequence (from the
>>>> cover letter) would be triggered.
>>>>
>>>> 1) UFFDIO_COPY
>>>> 2) MADV_DONTNEED
>>>>
>>>> So we would just handle the operation internally without a fallback.
>>>
>>> Note that I think there will be a slight difference on whole remap
>>> atomicity, on what happens if the page is modified after UFFDIO_COPY but
>>> before DONTNEED.
>>>
>>> UFFDIO_REMAP guarantees full atomicity when moving the page, IOW, threads
>>> can be updating the pages when ioctl(UFFDIO_REMAP), data won't get lost
>>> during movement, and it will generate a missing event after moved, with
>>> latest data showing up on dest.
>>>
>>> I'm not sure that means such a fallback is a problem, Suren may know
>>> better with the use case.
>>
>> Although there is no problem in using fallback with our use case but
>> as a user of userfaultfd, I'd suggest leaving it to the developer.
>> Failing with appropriate errno makes more sense. If handled in the
>> kernel, then the user may assume at the end of the operation that the
>> src vma is completely unmapped. And if not correctness issues, it
>> could lead to memory leaks.
> 
> I meant that in addition to the possibility of correctness issues due
> to lack of atomicity, it could also lead to memory leaks, as the user
> may assume that src vma is empty post-operation. IMHO, it's better to
> fail with errno so that the user would fix the code with necessary
> changes (like using DONTFORK, if forking).

Leaving the atomicity discussion out because I think this can just be 
handled (e.g., the src_vma would always be empty post-operation):

It might not necessarily be a good idea to only expose micro-operations 
to user space. If the user-space fallback will almost always be 
"UFFDIO_COPY+MADV_DONTNEED", then clearly the logical operation 
performed is moving data, ideally with zero-copy.

[as said as reply to Peter, one could still have magic flags for users 
that really want to detect when zero-copy is impossible]

With a logical MOVE API users like compaction [as given in the cover 
letter], not every such user has to eventually implement fallback paths.

But just my 2 cents, the UFFDIO_REMAP users probably can share what the 
exact use cases are and if fallbacks are required at all or if no-KSM + 
DONTFORK just does the trick.

-- 
Cheers,

David / dhildenb