Hi Peter, Please, see a few minor comments below.
Thanks, Alex On 3/10/21 11:22 PM, Peter Xu wrote: > Write-protect mode is supported starting from Linux 5.7. > > Signed-off-by: Peter Xu <pet...@redhat.com> > --- > man2/userfaultfd.2 | 103 ++++++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 101 insertions(+), 2 deletions(-) > > diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2 > index 555e37409..d1f9aad24 100644 > --- a/man2/userfaultfd.2 > +++ b/man2/userfaultfd.2 > @@ -78,6 +78,32 @@ all memory ranges that were registered with the object are > unregistered > and unread events are flushed. > .\" > .PP > +Userfaultfd supports two modes of registration: > +.TP > +.BR UFFDIO_REGISTER_MODE_MISSING " (since 4.10)" > +When registered with > +.B UFFDIO_REGISTER_MODE_MISSING > +mode, the userspace will receive a page fault message when a missing page is SC (see below) > +accessed. > +The faulted thread will be stopped from execution until the page fault is > +resolved from the userspace by either an > +.B UFFDIO_COPY > +or an > +.B UFFDIO_ZEROPAGE > +ioctl. > +.TP > +.BR UFFDIO_REGISTER_MODE_WP " (since 5.7)" > +When registered with > +.B UFFDIO_REGISTER_MODE_WP > +mode, the userspace will receive a page fault message when a write-protected > +page is written. Prefer breaking sentences at subordinate clauses rather than at random points: [ mode, the userspace will receive a page fault message when a write-protected page is written. ] There are other similar cases around the patch, marked with SC. > +The faulted thread will be stopped from execution until the userspace SC > +un-write-protect the page using an> +.B UFFDIO_WRITEPROTECT > +ioctl. > +.PP > +Multiple modes can be enabled at the same time for the same memory range. > +.PP > Since Linux 4.14, userfaultfd page fault message can selectively embed > faulting > thread ID information into the fault message. > One needs to enable this feature explicitly using the > @@ -144,6 +170,17 @@ 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 SC > +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 refer to "Userfaultfd write-protect mode" > section. > .SS Userfaultfd operation > After the userfaultfd object is created with > .BR userfaultfd (), > @@ -219,6 +256,64 @@ 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 5.7) > +Since Linux 5.7, userfaultfd supports write-protect mode. > +The user needs to first check availability of this feature using > +.B UFFDIO_API > +ioctl against the feature bit > +.B UFFD_FEATURE_PAGEFAULT_FLAG_WP > +before using this feature. > +.PP > +To register with userfaultfd write-protect mode, the user needs to initiate > the > +.B UFFDIO_REGISTER > +ioctl with mode > +.B 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 > +.B UFFDIO_REGISTER > +with the mode set to > +.BR "UFFDIO_REGISTER_MODE_MISSING | UFFDIO_REGISTER_MODE_WP" . > +When there is only > +.B UFFDIO_REGISTER_MODE_WP > +registered, the userspace will > +.I not > +receive any message when a missing page is written. > +Instead, the userspace will only receive a write-protect page fault message > +when an existing but write-protected page got written. > +.PP > +After the > +.B UFFDIO_REGISTER > +ioctl completed with > +.B UFFDIO_REGISTER_MODE_WP > +mode set, the user can write-protect any existing memory within the range > using SC (break at the comma better) > +the ioctl > +.B UFFDIO_WRITEPROTECT > +where > +.I uffdio_writeprotect.mode > +should be set to > +.BR UFFDIO_WRITEPROTECT_MODE_WP . > +.PP > +When a write-protect event happens, the userspace will receive a page fault SC (break at the comma better) > +message whose > +.I uffd_msg.pagefault.flags > +will be with > +.B UFFD_PAGEFAULT_FLAG_WP > +flag set. Note: since only writes can trigger such kind of fault, Break at the point above too. > +write-protect messages will always be with > +.B UFFD_PAGEFAULT_FLAG_WRITE > +bit set too along with bit > +.BR UFFD_PAGEFAULT_FLAG_WP . > +.PP > +To resolve a write-protection page fault, the user should initiate another > +.B UFFDIO_WRITEPROTECT > +ioctl, whose > +.I uffd_msg.pagefault.flags > +should have the flag > +.B UFFDIO_WRITEPROTECT_MODE_WP > +cleared upon the faulted page or range. > +.PP > +Write-protect mode only supports private anonymous memory. > .SS Reading from the userfaultfd structure > Each > .BR read (2) > @@ -364,8 +459,12 @@ flag (see > .BR ioctl_userfaultfd (2)) > and this flag is set, this a write fault; > otherwise it is a read fault. > -.\" > -.\" UFFD_PAGEFAULT_FLAG_WP is not yet supported. > +.TP > +.B UFFD_PAGEFAULT_FLAG_WP > +If the address is in a range that was registered with the > +.B UFFDIO_REGISTER_MODE_WP > +flag, when this bit is set it means it's a write-protect fault. Otherwise > it's > +a page missing fault. Break at the point. > .RE > .TP > .I pagefault.feat.pid > -- Alejandro Colomar Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/ http://www.alejandro-colomar.es/
