linux-kernel - Re: [PATCH v3 2/3] userfaultfd: UFFDIO

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Message-ID: <CAJuCfpH8oare_erzHuhiV0knbwVEmOzq6DnoywNQpOCAqJMucA@mail.gmail.com>
Date:   Mon, 23 Oct 2023 12:00:09 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     akpm@...ux-foundation.org, viro@...iv.linux.org.uk,
        brauner@...nel.org, shuah@...nel.org, aarcange@...hat.com,
        lokeshgidra@...gle.com, peterx@...hat.com, hughd@...gle.com,
        mhocko@...e.com, axelrasmussen@...gle.com, rppt@...nel.org,
        willy@...radead.org, Liam.Howlett@...cle.com, jannh@...gle.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 v3 2/3] userfaultfd: UFFDIO_MOVE uABI

On Mon, Oct 23, 2023 at 8:53 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 23.10.23 14:29, David Hildenbrand wrote:
> >> +
> >> +    /* Only allow remapping if both are mlocked or both aren't */
> >> +    if ((src_vma->vm_flags & VM_LOCKED) != (dst_vma->vm_flags & VM_LOCKED))
> >> +            return -EINVAL;
> >> +
> >> +    if (!(src_vma->vm_flags & VM_WRITE) || !(dst_vma->vm_flags & VM_WRITE))
> >> +            return -EINVAL;
> >
> > Why does one of both need VM_WRITE? If one really needs it, then the
> > destination (where we're moving stuff to).
>
> Just realized that we want both to be writable.
>
> If you have this in place, there is no need to use maybe*_mkwrite(), you
> can use the non-maybe variants.

Ack.

>
> I recall that for UFFDIO_COPY we even support PROT_NONE VMAs, is there
> any reason why we want to have different semantics here?

I don't think so. At least not for the single-mm case.

>
> --
> Cheers,
>
> David / dhildenb
>
> --
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