mm: khugepaged: avoid pointless allocation for "struct mm_slot"

In __khugepaged_enter(), if "mm->flags" with MMF_VM_HUGEPAGE bit is set,
the "mm_slot" will be released and return, so we can call mm_slot_alloc()
after test_and_set_bit().

Link: https://lkml.kernel.org/r/20230531095817.11012-1-xhao@linux.alibaba.comSigned-off-by: Xin Hao <xhao@linux.alibaba.com>
Reviewed-by: Andrew Morton <akpm@linux-foudation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/khugepaged.c

@@ -422,19 +422,17 @@ void __khugepaged_enter(struct mm_struct *mm)
 	struct mm_slot *slot;
 	int wakeup;
 
+	/* __khugepaged_exit() must not run from under us */
+	VM_BUG_ON_MM(hpage_collapse_test_exit(mm), mm);
+	if (unlikely(test_and_set_bit(MMF_VM_HUGEPAGE, &mm->flags)))
+		return;
+
 	mm_slot = mm_slot_alloc(mm_slot_cache);
 	if (!mm_slot)
 		return;
 
 	slot = &mm_slot->slot;
 
-	/* __khugepaged_exit() must not run from under us */
-	VM_BUG_ON_MM(hpage_collapse_test_exit(mm), mm);
-	if (unlikely(test_and_set_bit(MMF_VM_HUGEPAGE, &mm->flags))) {
-		mm_slot_free(mm_slot_cache, mm_slot);
-		return;
-	}
-
 	spin_lock(&khugepaged_mm_lock);
 	mm_slot_insert(mm_slots_hash, mm, slot);
 	/*