arm64: mm: Fix false positives in set_pte_at access/dirty race detection

Jiankang reports that our race detection in set_pte_at is firing when
copying the page tables in dup_mmap as a result of a fork(). In this
situation, the page table isn't actually live and so there is no way
that we can race with a concurrent update from the hardware page table
walker.

This patch reworks the race detection so that we require either the
mm to match the current active_mm (i.e. currently installed in our TTBR0)
or the mm_users count to be greater than 1, implying that the page table
could be live in another CPU. The mm_users check might still be racy,
but we'll avoid false positives and it's not realistic to validate that
all the necessary locks are held as part of this assertion.

Cc: Yisheng Xie <xieyisheng1@huawei.com>
Reported-by: Jiankang Chen <chenjiankang1@huawei.com>
Tested-by: Jiankang Chen <chenjiankang1@huawei.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/pgtable.h

@@ -42,6 +42,8 @@
 #include <asm/cmpxchg.h>
 #include <asm/fixmap.h>
 #include <linux/mmdebug.h>
+#include <linux/mm_types.h>
+#include <linux/sched.h>
 
 extern void __pte_error(const char *file, int line, unsigned long val);
 extern void __pmd_error(const char *file, int line, unsigned long val);
@@ -215,9 +217,6 @@ static inline void set_pte(pte_t *ptep, pte_t pte)
 	}
 }
 
-struct mm_struct;
-struct vm_area_struct;
-
 extern void __sync_icache_dcache(pte_t pteval, unsigned long addr);
 
 /*
@@ -246,7 +245,8 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
 	 * hardware updates of the pte (ptep_set_access_flags safely changes
 	 * valid ptes without going through an invalid entry).
 	 */
-	if (pte_valid(*ptep) && pte_valid(pte)) {
+	if (IS_ENABLED(CONFIG_DEBUG_VM) && pte_valid(*ptep) && pte_valid(pte) &&
+	   (mm == current->active_mm || atomic_read(&mm->mm_users) > 1)) {
 		VM_WARN_ONCE(!pte_young(pte),
 			     "%s: racy access flag clearing: 0x%016llx -> 0x%016llx",
 			     __func__, pte_val(*ptep), pte_val(pte));