Write-protect mode is supported starting from Linux 5.7. Signed-off-by: Peter Xu <peterx@xxxxxxxxxx> --- man2/userfaultfd.2 | 98 +++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 96 insertions(+), 2 deletions(-) diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2 index 0cd426a8a..426307bcf 100644 --- a/man2/userfaultfd.2 +++ b/man2/userfaultfd.2 @@ -78,6 +78,30 @@ 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 +.B UFFDIO_REGISTER_MODE_MISSING +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 +.B UFFDIO_REGISTER_MODE_WP +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 +168,16 @@ 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 (), @@ -219,6 +253,62 @@ 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 supports write-protect mode. The user needs to +first check availability of this feature using +.B 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 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 +.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 +.BR UFFDIO_WRITEPROTECT_MODE_WP +cleared upon the faulted page or range. +.PP +Currently, write-protect mode only supports private anonymous memory. .SS Reading from the userfaultfd structure Each .BR read (2) @@ -364,8 +454,12 @@ 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
