arm64: Improve error reporting on set_pte_at() checks

Currently the BUG_ON() checks do not give enough information about the
PTEs being set. This patch changes BUG_ON to WARN_ONCE and dumps the
values of the old and new PTEs. In addition, the checks are only made if
the new PTE entry is valid.
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: Ming Lei <tom.leiming@gmail.com>
Cc: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/pgtable.h

@@ -276,10 +276,14 @@ 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 (IS_ENABLED(CONFIG_DEBUG_VM) && IS_ENABLED(CONFIG_ARM64_HW_AFDBM) &&
-	    pte_valid(*ptep)) {
-		BUG_ON(!pte_young(pte));
-		BUG_ON(pte_write(*ptep) && !pte_dirty(pte));
+	if (IS_ENABLED(CONFIG_ARM64_HW_AFDBM) &&
+	    pte_valid(*ptep) && pte_valid(pte)) {
+		VM_WARN_ONCE(!pte_young(pte),
+			     "%s: racy access flag clearing: 0x%016llx -> 0x%016llx",
+			     __func__, pte_val(*ptep), pte_val(pte));
+		VM_WARN_ONCE(pte_write(*ptep) && !pte_dirty(pte),
+			     "%s: racy dirty state clearing: 0x%016llx -> 0x%016llx",
+			     __func__, pte_val(*ptep), pte_val(pte));
 	}
 
 	set_pte(ptep, pte);