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From: peterx@redhat.com
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	linux-mm@kvack.org
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Subject: [PATCH 09/13] mm/gup: Cache *pudp in follow_pud_mask()
Date: Tue, 19 Dec 2023 15:55:34 +0800
Message-ID: <20231219075538.414708-10-peterx@redhat.com>
In-Reply-To: <20231219075538.414708-1-peterx@redhat.com>
References: <20231219075538.414708-1-peterx@redhat.com>
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From: Peter Xu <peterx@redhat.com>

Introduce "pud_t pud" in the function, so the code won't dereference *pudp
multiple time.  Not only because that looks less straightforward, but also
because if the dereference really happened, it's not clear whether there
can be race to see different *pudp values if it's being modified at the
same time.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/gup.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 6c0d82fa8cc7..97e87b7a15c3 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -753,26 +753,27 @@ static struct page *follow_pud_mask(struct vm_area_struct *vma,
 				    unsigned int flags,
 				    struct follow_page_context *ctx)
 {
-	pud_t *pud;
+	pud_t *pudp, pud;
 	spinlock_t *ptl;
 	struct page *page;
 	struct mm_struct *mm = vma->vm_mm;
 
-	pud = pud_offset(p4dp, address);
-	if (pud_none(*pud))
+	pudp = pud_offset(p4dp, address);
+	pud = *pudp;
+	if (pud_none(pud))
 		return no_page_table(vma, flags, address);
-	if (pud_devmap(*pud)) {
-		ptl = pud_lock(mm, pud);
-		page = follow_devmap_pud(vma, address, pud, flags, &ctx->pgmap);
+	if (pud_devmap(pud)) {
+		ptl = pud_lock(mm, pudp);
+		page = follow_devmap_pud(vma, address, pudp, flags, &ctx->pgmap);
 		spin_unlock(ptl);
 		if (page)
 			return page;
 		return no_page_table(vma, flags, address);
 	}
-	if (unlikely(pud_bad(*pud)))
+	if (unlikely(pud_bad(pud)))
 		return no_page_table(vma, flags, address);
 
-	return follow_pmd_mask(vma, address, pud, flags, ctx);
+	return follow_pmd_mask(vma, address, pudp, flags, ctx);
 }
 
 static struct page *follow_p4d_mask(struct vm_area_struct *vma,
-- 
2.41.0