On Mon, Feb 1, 2021 at 12:07 PM Peter Xu <peterx@xxxxxxxxxx> 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 >
