Re: [PATCH v3 8/9] userfaultfd: update documentation to describe minor fault handling

[Axel Rasmussen <axelrasmussen@google.com>]

On Mon, Feb 1, 2021 at 12:07 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Thu, Jan 28, 2021 at 02:48:18PM -0800, Axel Rasmussen wrote:
> > Reword / reorganize things a little bit into "lists", so new features /
> > modes / ioctls can sort of just be appended.
> >
> > Describe how UFFDIO_REGISTER_MODE_MINOR and UFFDIO_CONTINUE can be used
> > to intercept and resolve minor faults. Make it clear that COPY and
> > ZEROPAGE are used for MISSING faults, whereas CONTINUE is used for MINOR
> > faults.
>
> Bare with me since I'm not native speaker.. but I'm pointing out things that
> reads odd to me.  Feel free to argue. :)

No worries, that is true for many people in the community. I'm happy
to reword to make things as clear as possible. :)

>
> [...]
>
> > +Resolving Userfaults
> > +--------------------
> > +
> > +There are three basic ways to resolve userfaults:
> > +
> > +- ``UFFDIO_COPY`` atomically copies some existing page contents from
> > +  userspace.
> > +
> > +- ``UFFDIO_ZEROPAGE`` atomically zeros the new page.
> > +
> > +- ``UFFDIO_CONTINUE`` maps an existing, previously-populated page.
> > +
> > +These operations are atomic in the sense that they guarantee nothing can
> > +see a half-populated page, since readers will keep userfaulting until the
> > +operation has finished.
> > +
> > +By default, these wake up userfaults blocked on the range in question.
> > +They support a ``UFFDIO_*_MODE_DONTWAKE`` ``mode`` flag, which indicates
> > +that waking will be done separately at some later time.
> > +
> > +Which of these are used depends on the kind of fault:
>
> Maybe:
>
> "We should choose the ioctl depending on the kind of the page fault, and what
>  we'd like to do with it:"
>
> ?
>
> > +
> > +- For ``UFFDIO_REGISTER_MODE_MISSING`` faults, a new page has to be
> > +  provided. This can be done with either ``UFFDIO_COPY`` or
>
> UFFDIO_ZEROPAGE does not need a new page.
>
> > +  ``UFFDIO_ZEROPAGE``. The default (non-userfaultfd) behavior would be to
> > +  provide a zero page, but in userfaultfd this is left up to userspace.
>
> "By default, kernel will provide a zero page for a missing fault.  With
>  userfaultfd, the userspace could decide which content to provide before the
>  faulted thread continues." ?
>
> > +
> > +- For ``UFFDIO_REGISTER_MODE_MINOR`` faults, an existing page already
>
> "page cache existed"?
>
> > +  exists. Userspace needs to ensure its contents are correct (if it needs
> > +  to be modified, by writing directly to the non-userfaultfd-registered
> > +  side of shared memory), and then issue ``UFFDIO_CONTINUE`` to resolve
> > +  the fault.
>
> "... Userspace can modify the page content before asking the faulted thread to
>  continue the fault with UFFDIO_CONTINUE ioctl." ?

I agree with all the comments; these areas can be clarified. I didn't
take the suggestions exactly as-is, but I did reword these parts in my
v4. Let me know if further changes would be useful.

>
> --
> Peter Xu
>