| Message ID | 20210304015947.517713-3-peterx@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | man2: udpate mm/userfaultfd manpages to latest | expand |
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@redhat.com> > --- > 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" > 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/ > +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. > +When there is only > +.BR 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 > +.BR UFFDIO_REGISTER > +ioctl completed with > +.BR UFFDIO_REGISTER_MODE_WP > +mode set, one can write-protect any existing memory within the range using the > +ioctl > +.BR 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 > +message whose > +.I uffd_msg.pagefault.flags > +will be with > +.BR UFFD_PAGEFAULT_FLAG_WP > +flag set. Note: since only writes can trigger such kind of fault, > +write-protect messages will always be with > +.BR UFFD_PAGEFAULT_FLAG_WRITE > +bit set too along with > +.BR UFFD_PAGEFAULT_FLAG_WP . > +.PP > +Currently, write-protect mode only supports private anonymous memory. > .SS Reading from the userfaultfd structure > Each > .BR read (2) > @@ -363,8 +443,12 @@ 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. > .RE > .TP > .I pagefault.feat.pid > -- > 2.26.2 >
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@redhat.com> > > --- > > 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,
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 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 +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. +When there is only +.BR 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 +.BR UFFDIO_REGISTER +ioctl completed with +.BR UFFDIO_REGISTER_MODE_WP +mode set, one can write-protect any existing memory within the range using the +ioctl +.BR 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 +message whose +.I uffd_msg.pagefault.flags +will be with +.BR UFFD_PAGEFAULT_FLAG_WP +flag set. Note: since only writes can trigger such kind of fault, +write-protect messages will always be with +.BR UFFD_PAGEFAULT_FLAG_WRITE +bit set too along with +.BR UFFD_PAGEFAULT_FLAG_WP . +.PP +Currently, write-protect mode only supports private anonymous memory. .SS Reading from the userfaultfd structure Each .BR read (2) @@ -363,8 +443,12 @@ 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. .RE .TP .I pagefault.feat.pid
Write-protect mode is supported starting from Linux 5.7. Signed-off-by: Peter Xu <peterx@redhat.com> --- man2/userfaultfd.2 | 88 ++++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 86 insertions(+), 2 deletions(-)