[RFC PATCH 2/6] mm: clone3: Add CLONE_COW_PGTABLE flag

[Chih-En Lin <shiyn.lin@gmail.com>]

Add CLONE_COW_PGTABLE flag to support clone3() system call to enable the
Copy-On-Write (COW) mechanism on the page table.

Signed-off-by: Chih-En Lin <shiyn.lin@gmail.com>
---
 include/uapi/linux/sched.h | 1 +
 kernel/fork.c              | 6 +++++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index 3bac0a8ceab2..3b92ff589e0f 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -36,6 +36,7 @@
 /* Flags for the clone3() syscall. */
 #define CLONE_CLEAR_SIGHAND 0x100000000ULL /* Clear any signal handler and reset to SIG_DFL. */
 #define CLONE_INTO_CGROUP 0x200000000ULL /* Clone into a specific cgroup given the right permissions. */
+#define CLONE_COW_PGTABLE 0x400000000ULL /* Copy-On-Write for page table */
 
 /*
  * cloning flags intersect with CSIGNAL so can be used with unshare and clone3
diff --git a/kernel/fork.c b/kernel/fork.c
index 35a3beff140b..08cf95201333 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2636,6 +2636,9 @@ pid_t kernel_clone(struct kernel_clone_args *args)
 			trace = 0;
 	}
 
+	if (clone_flags & CLONE_COW_PGTABLE)
+		set_bit(MMF_COW_PGTABLE, &current->mm->flags);
+
 	p = copy_process(NULL, trace, NUMA_NO_NODE, args);
 	add_latent_entropy();
 
@@ -2860,7 +2863,8 @@ static bool clone3_args_valid(struct kernel_clone_args *kargs)
 {
 	/* Verify that no unknown flags are passed along. */
 	if (kargs->flags &
-	    ~(CLONE_LEGACY_FLAGS | CLONE_CLEAR_SIGHAND | CLONE_INTO_CGROUP))
+	    ~(CLONE_LEGACY_FLAGS | CLONE_CLEAR_SIGHAND | CLONE_INTO_CGROUP |
+		    CLONE_COW_PGTABLE))
 		return false;
 
 	/*
-- 
2.36.1