Write-protect mode is supported starting from Linux 5.7. Signed-off-by: Peter Xu <peterx@xxxxxxxxxx> --- man2/userfaultfd.2 | 104 ++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 102 insertions(+), 2 deletions(-) diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2 index 555e37409..8ad4a71b5 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 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. +The faulted thread will be stopped from execution +until the userspace 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 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 +256,65 @@ 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 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 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, +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 +460,12 @@ 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
