[PATCH 03/22] mm: introduce MMF_KERNEL flag and set it for init_mm

[Kevin Brodsky]

mm code often needs to know whether some mm represents a kernel or
user address space. This is currently done by comparing the mm
pointer with &init_mm; besides not being particularly elegant, this
ignores the fact that other mm's (e.g. efi_mm) may also represent
parts of the kernel address space.

Introduce a new mm flag MMF_KERNEL and set it for init_mm.
Subsequent patches will use this flag to replace comparisons with
&init_mm. No functional change is introduced for now.

Assisted-by: Codex:GPT-5.5
Signed-off-by: Kevin Brodsky <kevin.brodsky at arm.com>
---
 include/linux/mm_types.h | 10 ++++++++++
 mm/init-mm.c             |  1 +
 2 files changed, 11 insertions(+)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index a2e0dc5892ff..7838ea3aac00 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1414,6 +1414,8 @@ struct mm_struct {
 	char flexible_array[] __aligned(__alignof__(unsigned long));
 };
 
+#define MM_FLAGS_INIT(flags)	{ .__mm_flags = { BITMAP_FROM_U64(flags) } }
+
 static inline bool mm_flags_test(int flag, const struct mm_struct *mm)
 {
 	return test_bit(flag, ACCESS_PRIVATE(&mm->flags, __mm_flags));
@@ -2001,6 +2003,9 @@ enum {
 #define MMF_TOPDOWN		31	/* mm searches top down by default */
 #define MMF_TOPDOWN_MASK	BIT(MMF_TOPDOWN)
 
+#define MMF_KERNEL		32	/* mm belongs to the kernel */
+#define MMF_KERNEL_MASK		BIT_ULL(MMF_KERNEL)
+
 #define MMF_INIT_LEGACY_MASK	(MMF_DUMP_FILTER_MASK |\
 				 MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK |\
 				 MMF_VM_MERGE_ANY_MASK | MMF_TOPDOWN_MASK)
@@ -2008,6 +2013,11 @@ enum {
 /* Legacy flags must fit within 32 bits. */
 static_assert((u64)MMF_INIT_LEGACY_MASK <= (u64)UINT_MAX);
 
+static inline bool mm_is_kernel(const struct mm_struct *mm)
+{
+	return mm && mm_flags_test(MMF_KERNEL, mm);
+}
+
 /*
  * Initialise legacy flags according to masks, propagating selected flags on
  * fork. Further flag manipulation can be performed by the caller.
diff --git a/mm/init-mm.c b/mm/init-mm.c
index 3e792aad7626..93773269bf87 100644
--- a/mm/init-mm.c
+++ b/mm/init-mm.c
@@ -34,6 +34,7 @@ struct mm_struct init_mm = {
 	.pgd		= swapper_pg_dir,
 	.mm_users	= ATOMIC_INIT(2),
 	.mm_count	= ATOMIC_INIT(1),
+	.flags		= MM_FLAGS_INIT(MMF_KERNEL_MASK),
 	.write_protect_seq = SEQCNT_ZERO(init_mm.write_protect_seq),
 	MMAP_LOCK_INITIALIZER(init_mm)
 	.page_table_lock =  __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),

-- 
2.51.2