linux-kernel - [PATCH v3 3/3] ioctl_userfaultfd.2: document new UFFDIO

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Message-ID: <20231017230110.3170850-4-axelrasmussen@google.com>
Date:   Tue, 17 Oct 2023 16:01:10 -0700
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Alejandro Colomar <alx@...nel.org>,
        Mike Rapoport <rppt@...nel.org>, Peter Xu <peterx@...hat.com>
Cc:     linux-man@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Axel Rasmussen <axelrasmussen@...gle.com>
Subject: [PATCH v3 3/3] ioctl_userfaultfd.2: document new UFFDIO_POISON ioctl

This is a new feature recently added to the kernel. So, document the new
ioctl the same way we do other UFFDIO_* ioctls.

Also note the corresponding new ioctl flag we can return in response to
a UFFDIO_REGISTER call.

Reviewed-by: Mike Rapoport (IBM) <rppt@...nel.org>
Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>
---
 man2/ioctl_userfaultfd.2 | 124 +++++++++++++++++++++++++++++++++++++--
 1 file changed, 118 insertions(+), 6 deletions(-)

diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
index 40528001e..a9131515c 100644
--- a/man2/ioctl_userfaultfd.2
+++ b/man2/ioctl_userfaultfd.2
@@ -405,6 +405,11 @@ operation is supported.
 The
 .B UFFDIO_CONTINUE
 operation is supported.
+.TP
+.B 1 << _UFFDIO_POISON
+The
+.B UFFDIO_POISON
+operation is supported.
 .PP
 This
 .BR ioctl (2)
@@ -886,6 +891,12 @@ does not equal the value that was specified in the
 .I range.len
 field.
 .TP
+.B EEXIST
+One or more pages were already mapped in the given range.
+.TP
+.B EFAULT
+No existing page could be found in the page cache for the given range.
+.TP
 .B EINVAL
 Either
 .I range.start
@@ -900,9 +911,6 @@ An invalid bit was specified in the
 .I mode
 field.
 .TP
-.B EEXIST
-One or more pages were already mapped in the given range.
-.TP
 .B ENOENT
 The faulting process has changed its virtual memory layout simultaneously with
 an outstanding
@@ -912,14 +920,118 @@ operation.
 .B ENOMEM
 Allocating memory needed to setup the page table mappings failed.
 .TP
-.B EFAULT
-No existing page could be found in the page cache for the given range.
-.TP
 .B ESRCH
 The faulting process has exited at the time of a
 .B UFFDIO_CONTINUE
 operation.
 .\"
+.SS UFFDIO_POISON
+(Since Linux 6.6.)
+Mark an address range as "poisoned".
+Future accesses to these addresses will raise a
+.B SIGBUS
+signal.
+Unlike
+.B MADV_HWPOISON
+this works by installing page table entries,
+rather than "really" poisoning the underlying physical pages.
+This means it only affects this particular address space.
+.PP
+The
+.I argp
+argument is a pointer to a
+.I uffdio_poison
+structure as shown below:
+.PP
+.in +4n
+.EX
+struct uffdio_poison {
+	struct uffdio_range range;
+	                /* Range to install poison PTE markers in */
+	__u64 mode;     /* Flags controlling the behavior of poison */
+	__s64 updated;  /* Number of bytes poisoned, or negated error */
+};
+.EE
+.in
+.PP
+The following value may be bitwise ORed in
+.I mode
+to change the behavior of the
+.B UFFDIO_POISON
+operation:
+.TP
+.B UFFDIO_POISON_MODE_DONTWAKE
+Do not wake up the thread that waits for page-fault resolution.
+.PP
+The
+.I updated
+field is used by the kernel
+to return the number of bytes that were actually poisoned,
+or an error in the same manner as
+.BR UFFDIO_COPY .
+If the value returned in the
+.I updated
+field doesn't match the value that was specified in
+.IR range.len ,
+the operation fails with the error
+.BR EAGAIN .
+The
+.I updated
+field is output-only;
+it is not read by the
+.B UFFDIO_POISON
+operation.
+.PP
+This
+.BR ioctl (2)
+operation returns 0 on success.
+In this case,
+the entire area was poisoned.
+On error, \-1 is returned and
+.I errno
+is set to indicate the error.
+Possible errors include:
+.TP
+.B EAGAIN
+The number of bytes mapped
+(i.e., the value returned in the
+.I updated
+field)
+does not equal the value that was specified in the
+.I range.len
+field.
+.TP
+.B EINVAL
+Either
+.I range.start
+or
+.I range.len
+was not a multiple of the system page size; or
+.I range.len
+was zero; or the range specified was invalid.
+.TP
+.B EINVAL
+An invalid bit was specified in the
+.I mode
+field.
+.TP
+.B EEXIST
+One or more pages were already mapped in the given range.
+.TP
+.B ENOENT
+The faulting process has changed its virtual memory layout simultaneously with
+an outstanding
+.B UFFDIO_POISON
+operation.
+.TP
+.B ENOMEM
+Allocating memory for page table entries failed.
+.TP
+.B ESRCH
+The faulting process has exited at the time of a
+.B UFFDIO_POISON
+operation.
+.\"
 .SH RETURN VALUE
 See descriptions of the individual operations, above.
 .SH ERRORS
-- 
2.42.0.655.g421f12c284-goog