[PATCH v2 03/13] mm: add mk_vma_flags() bitmap flag macro helper

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

This patch introduces the mk_vma_flags() macro helper to allow easy
manipulation of VMA flags utilising the new bitmap representation
implemented of VMA flags defined by the vma_flags_t type.

It is a variadic macro which provides a bitwise-or'd representation of all
of each individual VMA flag specified.

Note that, while we maintain VM_xxx flags for backwards compatibility until
the conversion is complete, we define VMA flags of type vma_flag_t using
VMA_xxx_BIT to avoid confusing the two.

This helper macro therefore can be used thusly:

vma_flags_t flags = mk_vma_flags(VMA_READ_BIT, VMA_WRITE_BIT);

We allow for up to 5 flags to specified at a time which should accommodate
all current kernel uses of combined VMA flags.

Testing has demonstrated that the compiler optimises this code such that it
generates the same assembly utilising this macro as it does if the flags
were specified manually, for instance:

vma_flags_t get_flags(void)
{
	return mk_vma_flags(VMA_READ_BIT, VMA_WRITE_BIT, VMA_EXEC_BIT);
}

Generates the same code as:

vma_flags_t get_flags(void)
{
	vma_flags_t flags;

	vma_flags_clear_all(&flags);
	vma_flag_set(&flags, VMA_READ_BIT);
	vma_flag_set(&flags, VMA_WRITE_BIT);
	vma_flag_set(&flags, VMA_EXEC_BIT);

	return flags;
}

And:

vma_flags_t get_flags(void)
{
	vma_flags_t flags;
	unsigned long *bitmap = ACCESS_PRIVATE(&flags, __vma_flags);

	*bitmap = 1UL << (__force int)VMA_READ_BIT;
	*bitmap |= 1UL << (__force int)VMA_WRITE_BIT;
	*bitmap |= 1UL << (__force int)VMA_EXEC_BIT;

	return flags;
}

That is:

get_flags:
        movl    $7, %eax
        ret

Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 include/linux/mm.h | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index e0d31238097c..32c3b5347dc6 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2,6 +2,7 @@
 #ifndef _LINUX_MM_H
 #define _LINUX_MM_H
 
+#include <linux/args.h>
 #include <linux/errno.h>
 #include <linux/mmdebug.h>
 #include <linux/gfp.h>
@@ -1026,6 +1027,38 @@ static inline bool vma_test_atomic_flag(struct vm_area_struct *vma, vma_flag_t b
 	return false;
 }
 
+/* Set an individual VMA flag in flags, non-atomically. */
+static inline void vma_flag_set(vma_flags_t *flags, vma_flag_t bit)
+{
+	unsigned long *bitmap = flags->__vma_flags;
+
+	__set_bit((__force int)bit, bitmap);
+}
+
+static inline vma_flags_t __mk_vma_flags(size_t count, const vma_flag_t *bits)
+{
+	vma_flags_t flags;
+	int i;
+
+	vma_flags_clear_all(&flags);
+	for (i = 0; i < count; i++)
+		vma_flag_set(&flags, bits[i]);
+	return flags;
+}
+
+/*
+ * Helper macro which bitwise-or combines the specified input flags into a
+ * vma_flags_t bitmap value. E.g.:
+ *
+ * vma_flags_t flags = mk_vma_flags(VMA_IO_BIT, VMA_PFNMAP_BIT,
+ * 		VMA_DONTEXPAND_BIT, VMA_DONTDUMP_BIT);
+ *
+ * The compiler cleverly optimises away all of the work and this ends up being
+ * equivalent to aggregating the values manually.
+ */
+#define mk_vma_flags(...) __mk_vma_flags(COUNT_ARGS(__VA_ARGS__), \
+					 (const vma_flag_t []){__VA_ARGS__})
+
 static inline void vma_set_anonymous(struct vm_area_struct *vma)
 {
 	vma->vm_ops = NULL;
-- 
2.52.0