[PATCH v3] userfaultfd: provide pid in userfault msg

[Alexey Perevalov <a.perevalov@samsung.com>]

It could be useful for calculating downtime during
postcopy live migration per vCPU. Side observer or application itself
will be informed about proper task's sleep during userfaultfd
processing.

Process's thread id is being provided when user requeste it
by setting UFFD_FEATURE_THREAD_ID bit into uffdio_api.features.

Signed-off-by: Alexey Perevalov <a.perevalov@samsung.com>
---
 fs/userfaultfd.c                 | 8 ++++++--
 include/uapi/linux/userfaultfd.h | 8 +++++++-
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 24fd7e0..14c30d4 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -180,7 +180,8 @@ static inline void msg_init(struct uffd_msg *msg)
 
 static inline struct uffd_msg userfault_msg(unsigned long address,
 					    unsigned int flags,
-					    unsigned long reason)
+					    unsigned long reason,
+					    unsigned int features)
 {
 	struct uffd_msg msg;
 	msg_init(&msg);
@@ -204,6 +205,8 @@ static inline struct uffd_msg userfault_msg(unsigned long address,
 		 * write protect fault.
 		 */
 		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WP;
+	if (features & UFFD_FEATURE_THREAD_ID)
+		msg.arg.pagefault.ptid = task_pid_vnr(current);
 	return msg;
 }
 
@@ -408,7 +411,8 @@ int handle_userfault(struct vm_fault *vmf, unsigned long reason)
 
 	init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
 	uwq.wq.private = current;
-	uwq.msg = userfault_msg(vmf->address, vmf->flags, reason);
+	uwq.msg = userfault_msg(vmf->address, vmf->flags, reason,
+			ctx->features);
 	uwq.ctx = ctx;
 	uwq.waken = false;
 
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index 819e235..ff8d0d2 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -24,7 +24,8 @@
 			   UFFD_FEATURE_EVENT_REMOVE |	\
 			   UFFD_FEATURE_EVENT_UNMAP |		\
 			   UFFD_FEATURE_MISSING_HUGETLBFS |	\
-			   UFFD_FEATURE_MISSING_SHMEM)
+			   UFFD_FEATURE_MISSING_SHMEM |		\
+			   UFFD_FEATURE_THREAD_ID)
 #define UFFD_API_IOCTLS				\
 	((__u64)1 << _UFFDIO_REGISTER |		\
 	 (__u64)1 << _UFFDIO_UNREGISTER |	\
@@ -83,6 +84,7 @@ struct uffd_msg {
 		struct {
 			__u64	flags;
 			__u64	address;
+			__u32   ptid;
 		} pagefault;
 
 		struct {
@@ -158,6 +160,9 @@ struct uffdio_api {
 	 * UFFD_FEATURE_MISSING_SHMEM works the same as
 	 * UFFD_FEATURE_MISSING_HUGETLBFS, but it applies to shmem
 	 * (i.e. tmpfs and other shmem based APIs).
+	 *
+	 * UFFD_FEATURE_THREAD_ID pid of the page faulted task_struct will
+	 * be returned, if feature is not requested 0 will be returned.
 	 */
 #define UFFD_FEATURE_PAGEFAULT_FLAG_WP		(1<<0)
 #define UFFD_FEATURE_EVENT_FORK			(1<<1)
@@ -166,6 +171,7 @@ struct uffdio_api {
 #define UFFD_FEATURE_MISSING_HUGETLBFS		(1<<4)
 #define UFFD_FEATURE_MISSING_SHMEM		(1<<5)
 #define UFFD_FEATURE_EVENT_UNMAP		(1<<6)
+#define UFFD_FEATURE_THREAD_ID			(1<<7)
 	__u64 features;
 
 	__u64 ioctls;
-- 
2.7.4

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