On Thu, Mar 04, 2021 at 08:46:59AM +0200, Mike Rapoport wrote: > On Wed, Mar 03, 2021 at 08:59:45PM -0500, Peter Xu wrote: > > Write-protect mode is supported starting from Linux 5.7. > > > > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx> > > --- > > man2/userfaultfd.2 | 88 ++++++++++++++++++++++++++++++++++++++++++++-- > > 1 file changed, 86 insertions(+), 2 deletions(-) > > > > diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2 > > index 2d14effc6..8e1602d62 100644 > > --- a/man2/userfaultfd.2 > > +++ b/man2/userfaultfd.2 > > @@ -78,6 +78,28 @@ all memory ranges that were registered with the object are unregistered > > and unread events are flushed. > > .\" > > .PP > > +Currently, userfaultfd supports two modes of registration: > > +.TP > > +.BR UFFDIO_REGISTER_MODE_MISSING > > +When registered with > > +.BR UFFDIO_REGISTER_MODE_MISSING > > +mode, the userspace will receive a page fault message when a missing page is > > +accessed. The faulted thread will be stopped from execution until the page > > +fault is resolved from the userspace by either an > > +.BR UFFDIO_COPY > > +or an > > +.BR UFFDIO_ZEROPAGE > > +ioctl. > > +.TP > > +.BR UFFDIO_REGISTER_MODE_WP > > +When registered with > > +.BR UFFDIO_REGISTER_MODE_WP > > +mode, the userspace will receive a page fault message when a write-protected > > +page is written. The faulted thread will be stopped from execution until the > > +userspace un-write-protect the page using an > > +.BR UFFDIO_WRITEPROTECT > > +ioctl. > > +.PP > > I'd add a sentence about combining the modes together. Something like > > "Both modes can be enabled together for the same memory range" I mentioned it below [1]. However I agree it's indeed making more sense to mention it when listing the modes, especially knowing that the 3rd minor mode is coming. I think I'll keep both, assuming a bit more verbose is still acceptable in man pages, but changed to: "Multiple modes can be enabled at the same time for the same memory range." > > > Since Linux 4.14, userfaultfd page fault message can selectively embed fault > > thread ID information into the fault message. One needs to enable this feature > > explicitly using the > > @@ -143,6 +165,16 @@ single threaded non-cooperative userfaultfd manager implementations. > > .\" and limitations remaining in 4.11 > > .\" Maybe it's worth adding a dedicated sub-section... > > .\" > > +.PP > > +Starting from Linux 5.7, userfaultfd is able to do synchronous page dirty > > +tracking using the new write-protection register mode. One should check > > +against the feature bit > > +.B UFFD_FEATURE_PAGEFAULT_FLAG_WP > > +before using this feature. Similar to the original userfaultfd missing mode, > > +the write-protect mode will generate an userfaultfd message when the protected > > +page is written. The user needs to resolve the page fault by unprotecting the > > +faulted page and kick the faulted thread to continue. For more information, > > +please read the "Userfaultfd write-protect mode" section below. > > .SS Userfaultfd operation > > After the userfaultfd object is created with > > .BR userfaultfd (), > > @@ -218,6 +250,54 @@ userfaultfd can be used only with anonymous private memory mappings. > > Since Linux 4.11, > > userfaultfd can be also used with hugetlbfs and shared memory mappings. > > .\" > > +.SS Userfaultfd write-protect mode > > +Since Linux 5.7, userfaultfd started to support write-protect mode. The user > > Maybe s/started to support/supports/ Sure. > > > +needs to first check availability of this feature using > > +.BR UFFDIO_API > > +ioctl against the feature bit > > +.BR UFFD_FEATURE_PAGEFAULT_FLAG_WP . > > +.PP > > +To register with userfaultfd write-protect mode, the user needs to send the > > +.BR UFFDIO_REGISTER > > +ioctl with mode > > +.BR UFFDIO_REGISTER_MODE_WP > > +set. Note that it's legal to monitor the same memory range with multiple > > +modes. For example, the user can do > > +.BR UFFDIO_REGISTER > > +with the mode set to > > +.BR UFFDIO_REGISTER_MODE_MISSING\ |\ UFFDIO_REGISTER_MODE_WP. [1] Thanks, -- Peter Xu
