[PATCH RFC v2 2/4] mm/ptshare: Add flag MAP_SHARED_PT to mmap()

[Khalid Aziz <khalid.aziz@oracle.com>]

When a process mmaps a file with MAP_SHARED flag, it is possible
that any other processes mmaping the same file with MAP_SHARED
flag with same permissions could share the page table entries as
well instead of creating duplicate entries. This patch introduces a
new flag MAP_SHARED_PT for mmap() which a process can use to hint
that it can share page tables with other processes using the same
mapping.

Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com>
---
 include/uapi/asm-generic/mman-common.h |  1 +
 mm/mmap.c                              | 16 ++++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h
index 6ce1f1ceb432..4d23456b5915 100644
--- a/include/uapi/asm-generic/mman-common.h
+++ b/include/uapi/asm-generic/mman-common.h
@@ -29,6 +29,7 @@
 #define MAP_HUGETLB		0x040000	/* create a huge page mapping */
 #define MAP_SYNC		0x080000 /* perform synchronous page faults for the mapping */
 #define MAP_FIXED_NOREPLACE	0x100000	/* MAP_FIXED which doesn't unmap underlying mapping */
+#define MAP_SHARED_PT		0x200000	/* Shared page table mappings */
 
 #define MAP_UNINITIALIZED 0x4000000	/* For anonymous mmap, memory could be
 					 * uninitialized */
diff --git a/mm/mmap.c b/mm/mmap.c
index d5475fbf5729..8b46d465f8d4 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1197,6 +1197,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 	struct mm_struct *mm = current->mm;
 	vm_flags_t vm_flags;
 	int pkey = 0;
+	int ptshare = 0;
 
 	validate_mm(mm);
 	*populate = 0;
@@ -1234,6 +1235,21 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 	if (mm->map_count > sysctl_max_map_count)
 		return -ENOMEM;
 
+	/*
+	 * If MAP_SHARED_PT is set, MAP_SHARED or MAP_SHARED_VALIDATE must
+	 * be set as well
+	 */
+	if (flags & MAP_SHARED_PT) {
+#if VM_SHARED_PT
+		if (flags & (MAP_SHARED | MAP_SHARED_VALIDATE))
+			ptshare = 1;
+		else
+			return -EINVAL;
+#else
+		return -EINVAL;
+#endif
+	}
+
 	/* Obtain the address to map to. we verify (or select) it and ensure
 	 * that it represents a valid section of the address space.
 	 */
-- 
2.37.2