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To:     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 28.09.23 20:34, Peter Xu wrote:
> On Thu, Sep 28, 2023 at 07:51:18PM +0200, David Hildenbrand wrote:
>> On 28.09.23 19:21, Peter Xu wrote:
>>> On Thu, Sep 28, 2023 at 07:05:40PM +0200, David Hildenbrand wrote:
>>>> As described as reply to v1, without fork() and KSM, the PAE bit should
>>>> stick around. If that's not the case, we should investigate why.
>>>>
>>>> If we ever support the post-fork case (which the comment above remap_pages()
>>>> excludes) we'll need good motivation why we'd want to make this
>>>> overly-complicated feature even more complicated.
>>>
>>> The problem is DONTFORK is only a suggestion, but not yet restricted.  If
>>> someone reaches on top of some !PAE page on src it'll never gonna proceed
>>> and keep failing, iiuc.
>>
>> Yes. It won't work if you fork() and not use DONTFORK on the src VMA. We
>> should document that as a limitation.
>>
>> For example, you could return an error to the user that can just call
>> UFFDIO_COPY. (or to the UFFDIO_COPY from inside uffd code, but that's
>> probably ugly as well).
> 
> We could indeed provide some special errno perhaps upon the PAE check, then
> document it explicitly in the man page and suggest resolutions (like
> DONTFORK) when user hit it.
> 

Maybe it might be reasonable to consider an operation that moves the 
page, even if it might do an internal copy. UFFDIO_MOVE might be a 
better name for something like that.

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.

The recommendation to the user to make the overall operation as fast as 
possible would be to not use KSM, to avoid fork(), or to use 
MADV_DONTFORK when fork() must be used.


-- 
Cheers,

David / dhildenb