linux-kernel - Re: [PATCH v2 2/3] userfaultfd: UFFDIO

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Date:   Mon, 2 Oct 2023 19:36:01 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Jann Horn <jannh@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        akpm@...ux-foundation.org, viro@...iv.linux.org.uk,
        brauner@...nel.org, shuah@...nel.org, aarcange@...hat.com,
        lokeshgidra@...gle.com, hughd@...gle.com, mhocko@...e.com,
        axelrasmussen@...gle.com, rppt@...nel.org, willy@...radead.org,
        Liam.Howlett@...cle.com, zhangpeng362@...wei.com,
        bgeffon@...gle.com, kaleshsingh@...gle.com, ngeoffray@...gle.com,
        jdduke@...gle.com, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI

On 02.10.23 19:33, David Hildenbrand wrote:
> On 02.10.23 17:21, Peter Xu 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.
> 
> If the page is writable (implies PAE), we can always move it. If it is
> R/O, it cannot change before we get a page fault and grab the PT lock
> (well, and page lock).
> 
> So I think something atomic can be implemented without too much issues.
> 
>>
>> 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.
> 
> If the page has to be copied, grab a reference and unmap it, then copy
> it and map it into the new process. Should be doable and handle all
> kinds of situations just fine.
> 
> Just throwing out ideas to get a less low-level interface.
> 
> [if one really wants to get notified when one cannot move without a
> copy, one could have a flag for such power users to control the behavior]
> 

[of course, if someone would have a GUP-pin on such a page, the page 
exchange would be observable. Just have to documented the UFFDIO_MOVE 
semantics properly]

-- 
Cheers,

David / dhildenb