linux-kernel - [PATCH v5 05/10] userfaultfd/shmem: advertise shmem minor fault support

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Date:   Tue, 27 Apr 2021 15:52:39 -0700
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Joe Perches <joe@...ches.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Peter Xu <peterx@...hat.com>, Shaohua Li <shli@...com>,
        Shuah Khan <shuah@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Wang Qing <wangqing@...o.com>
Cc:     linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-mm@...ck.org, Axel Rasmussen <axelrasmussen@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: [PATCH v5 05/10] userfaultfd/shmem: advertise shmem minor fault support

Now that the feature is fully implemented (the faulting path hooks exist
so userspace is notified, and the ioctl to resolve such faults is
available), advertise this as a supported feature.

Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>
---
 Documentation/admin-guide/mm/userfaultfd.rst | 3 ++-
 fs/userfaultfd.c                             | 3 ++-
 include/uapi/linux/userfaultfd.h             | 7 ++++++-
 3 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst
index 3aa38e8b8361..6528036093e1 100644
--- a/Documentation/admin-guide/mm/userfaultfd.rst
+++ b/Documentation/admin-guide/mm/userfaultfd.rst
@@ -77,7 +77,8 @@ events, except page fault notifications, may be generated:
 
 - ``UFFD_FEATURE_MINOR_HUGETLBFS`` indicates that the kernel supports
   ``UFFDIO_REGISTER_MODE_MINOR`` registration for hugetlbfs virtual memory
-  areas.
+  areas. ``UFFD_FEATURE_MINOR_SHMEM`` is the analogous feature indicating
+  support for shmem virtual memory areas.
 
 The userland application should set the feature flags it intends to use
 when invoking the ``UFFDIO_API`` ioctl, to request that those features be
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 468556fb04a9..9f3b8684cf3c 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1940,7 +1940,8 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
 	/* report all available features and ioctls to userland */
 	uffdio_api.features = UFFD_API_FEATURES;
 #ifndef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
-	uffdio_api.features &= ~UFFD_FEATURE_MINOR_HUGETLBFS;
+	uffdio_api.features &=
+		~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM);
 #endif
 	uffdio_api.ioctls = UFFD_API_IOCTLS;
 	ret = -EFAULT;
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index bafbeb1a2624..159a74e9564f 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -31,7 +31,8 @@
 			   UFFD_FEATURE_MISSING_SHMEM |		\
 			   UFFD_FEATURE_SIGBUS |		\
 			   UFFD_FEATURE_THREAD_ID |		\
-			   UFFD_FEATURE_MINOR_HUGETLBFS)
+			   UFFD_FEATURE_MINOR_HUGETLBFS |	\
+			   UFFD_FEATURE_MINOR_SHMEM)
 #define UFFD_API_IOCTLS				\
 	((__u64)1 << _UFFDIO_REGISTER |		\
 	 (__u64)1 << _UFFDIO_UNREGISTER |	\
@@ -185,6 +186,9 @@ struct uffdio_api {
 	 * UFFD_FEATURE_MINOR_HUGETLBFS indicates that minor faults
 	 * can be intercepted (via REGISTER_MODE_MINOR) for
 	 * hugetlbfs-backed pages.
+	 *
+	 * UFFD_FEATURE_MINOR_SHMEM indicates the same support as
+	 * UFFD_FEATURE_MINOR_HUGETLBFS, but for shmem-backed pages instead.
 	 */
 #define UFFD_FEATURE_PAGEFAULT_FLAG_WP		(1<<0)
 #define UFFD_FEATURE_EVENT_FORK			(1<<1)
@@ -196,6 +200,7 @@ struct uffdio_api {
 #define UFFD_FEATURE_SIGBUS			(1<<7)
 #define UFFD_FEATURE_THREAD_ID			(1<<8)
 #define UFFD_FEATURE_MINOR_HUGETLBFS		(1<<9)
+#define UFFD_FEATURE_MINOR_SHMEM		(1<<10)
 	__u64 features;
 
 	__u64 ioctls;
-- 
2.31.1.498.g6c1eba8ee3d-goog