[PATCH 2/5] userfaultfd.2: Add UFFD_FEATURE_THREAD_ID docs

Alejandro Colomar
2021-04-05T13:13:18Z

From: Peter Xu <peterx@redhat.com>

UFFD_FEATURE_THREAD_ID is supported since Linux 4.14.

Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
[alx: srcfix]
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
---
man2/userfaultfd.2 | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
index fa1b6514f..8245253c3 100644
--- a/man2/userfaultfd.2
+++ b/man2/userfaultfd.2
@@ -77,6 +77,13 @@ When the last file descriptor referring to a userfaultfd object is closed,
all memory ranges that were registered with the object are unregistered
and unread events are flushed.
.\"
+.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
+.B UFFD_FEATURE_THREAD_ID
+feature bit when initializing the userfaultfd context.
+By default, thread ID reporting is disabled.
.SS Usage
The userfaultfd mechanism is designed to allow a thread in a multithreaded
program to perform user-space paging for the other threads in the process.
@@ -229,6 +236,9 @@ struct uffd_msg {
struct {
__u64 flags; /* Flags describing fault */
__u64 address; /* Faulting address */
+ union {
+ __u32 ptid; /* Thread ID of the fault */
+ } feat;
} pagefault;

struct { /* Since Linux 4.11 */
@@ -358,6 +368,9 @@ otherwise it is a read fault.
.\" UFFD_PAGEFAULT_FLAG_WP is not yet supported.
.RE
.TP
+.I pagefault.feat.pid
+The thread ID that triggered the page fault.
+.TP
.I fork.ufd
The file descriptor associated with the userfault object
created for the child created by
--
2.31.0