linux-kernel - [PATCH] mm/pgtable-generic.c: optimize the VM_BUG_ON condition in pmdp_huge_clear

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Message-ID: <20210203084137.25522-1-linmiaohe@huawei.com>
Date:   Wed, 3 Feb 2021 03:41:37 -0500
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     <akpm@...ux-foundation.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <linmiaohe@...wei.com>
Subject: [PATCH] mm/pgtable-generic.c: optimize the VM_BUG_ON condition in pmdp_huge_clear_flush()

The developer will have trouble figuring out why the BUG actually triggered
when there is a complex expression in the VM_BUG_ON. Because we can only
identify the condition triggered BUG via line number provided by VM_BUG_ON.
Optimize this by spliting such a complex expression into two simple
conditions.

Suggested-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
---
 mm/pgtable-generic.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index fa1375f3e3b2..c2210e1cdb51 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -135,8 +135,9 @@ pmd_t pmdp_huge_clear_flush(struct vm_area_struct *vma, unsigned long address,
 {
 	pmd_t pmd;
 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
-	VM_BUG_ON(!pmd_present(*pmdp) || (!pmd_trans_huge(*pmdp) &&
-					  !pmd_devmap(*pmdp)));
+	VM_BUG_ON(!pmd_present(*pmdp));
+	/* Below assumes pmd_present() is true */
+	VM_BUG_ON(!pmd_trans_huge(*pmdp) && !pmd_devmap(*pmdp));
 	pmd = pmdp_huge_get_and_clear(vma->vm_mm, address, pmdp);
 	flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
 	return pmd;
-- 
2.19.1