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. :) [...] > +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." ? -- Peter Xu
