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From:   Peter Xu <peterx@redhat.com>
To:     linux-man@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Cc:     Andrea Arcangeli <aarcange@redhat.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Alejandro Colomar <alx.manpages@gmail.com>, peterx@redhat.com,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: [PATCH v4 1/4] userfaultfd.2: Add UFFD_FEATURE_THREAD_ID docs
Date:   Mon, 22 Mar 2021 18:08:45 -0400
Message-Id: <20210322220848.52162-2-peterx@redhat.com>
In-Reply-To: <20210322220848.52162-1-peterx@redhat.com>
References: <20210322220848.52162-1-peterx@redhat.com>
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UFFD_FEATURE_THREAD_ID is supported since Linux 4.14.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 man2/userfaultfd.2 | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
index e7dc9f813..555e37409 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
+.BR UFFD_FEATURE_THREAD_ID
+feature bit when initializing the userfaultfd context.
+By default, thread ID reporting is diabled.
 .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
-- 
2.26.2