mm/mmu_notifier: mmu_notifier_range_update_to_read_only() helper

Helper to test if a range is updated to read only (it is still valid to
read from the range).  This is useful for device driver or anyone who wish
to optimize out update when they know that they already have the range map
read only.

Link: http://lkml.kernel.org/r/20190326164747.24405-9-jglisse@redhat.com
Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: Jason Gunthorpe <jgg@mellanox.com>
Cc: Ross Zwisler <zwisler@kernel.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krcmar <rkrcmar@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Christian Koenig <christian.koenig@amd.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmu_notifier.h

@@ -259,6 +259,8 @@ extern void __mmu_notifier_invalidate_range_end(struct mmu_notifier_range *r,
 				  bool only_end);
 extern void __mmu_notifier_invalidate_range(struct mm_struct *mm,
 				  unsigned long start, unsigned long end);
+extern bool
+mmu_notifier_range_update_to_read_only(const struct mmu_notifier_range *range);
 
 static inline bool
 mmu_notifier_range_blockable(const struct mmu_notifier_range *range)
@@ -568,6 +570,8 @@ static inline void mmu_notifier_mm_destroy(struct mm_struct *mm)
 {
 }
 
+#define mmu_notifier_range_update_to_read_only(r) false
+
 #define ptep_clear_flush_young_notify ptep_clear_flush_young
 #define pmdp_clear_flush_young_notify pmdp_clear_flush_young
 #define ptep_clear_young_notify ptep_test_and_clear_young

mm/mmu_notifier.c

@@ -395,3 +395,13 @@ void mmu_notifier_unregister_no_release(struct mmu_notifier *mn,
 	mmdrop(mm);
 }
 EXPORT_SYMBOL_GPL(mmu_notifier_unregister_no_release);
+
+bool
+mmu_notifier_range_update_to_read_only(const struct mmu_notifier_range *range)
+{
+	if (!range->vma || range->event != MMU_NOTIFY_PROTECTION_VMA)
+		return false;
+	/* Return true if the vma still have the read flag set. */
+	return range->vma->vm_flags & VM_READ;
+}
+EXPORT_SYMBOL_GPL(mmu_notifier_range_update_to_read_only);