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.com
Signed-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>

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 6b9d39d65b7303e42767ee856e8c8ec21633477d..3649ba12a2355fe9f0efec3df8e552a835fd7786 100644 (file)
--- a/mm/khugepaged.c
+++ b/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);
        /*