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Date: Mon, 29 Mar 2021 18:18:31 -0400
From: Peter Xu <peterx@...hat.com>
To: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
linux-man@...r.kernel.org
Cc: Axel Rasmussen <axelrasmussen@...gle.com>, peterx@...hat.com,
Nadav Amit <nadav.amit@...il.com>,
Mike Rapoport <rppt@...ux.vnet.ibm.com>,
Alejandro Colomar <alx.manpages@...il.com>,
Andrea Arcangeli <aarcange@...hat.com>,
Michael Kerrisk <mtk.manpages@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH v5 2/4] userfaultfd.2: Add write-protect mode
Write-protect mode is supported starting from Linux 5.7.
Acked-by: Mike Rapoport <rppt@...ux.vnet.ibm.com>
Signed-off-by: Peter Xu <peterx@...hat.com>
---
man2/userfaultfd.2 | 108 +++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 104 insertions(+), 4 deletions(-)
diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
index 5c41e4816..474294c3d 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 write-unprotect 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
@@ -107,7 +133,7 @@ the process that monitors userfaultfd and handles page faults
needs to be aware of the changes in the virtual memory layout
of the faulting process to avoid memory corruption.
.PP
-Starting from Linux 4.11,
+Since Linux 4.11,
userfaultfd can also notify the fault-handling threads about changes
in the virtual memory layout of the faulting process.
In addition, if the faulting process invokes
@@ -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
+Since Linux 5.7, userfaultfd is able to do
+synchronous page dirty tracking using the new write-protect 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 (),
@@ -179,7 +216,7 @@ or
.BR ioctl (2)
operations to resolve the page fault.
.PP
-Starting from Linux 4.14, if the application sets the
+Since Linux 4.14, if the application sets the
.B UFFD_FEATURE_SIGBUS
feature bit using the
.B UFFDIO_API
@@ -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
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