linux-kernel - [PATCH v2 09/13] mm/gup: Cache *pudp in follow_pud

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Date: Wed,  3 Jan 2024 17:14:19 +0800
From: peterx@...hat.com
To: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Cc: James Houghton <jthoughton@...gle.com>,
	David Hildenbrand <david@...hat.com>,
	"Kirill A . Shutemov" <kirill@...temov.name>,
	Yang Shi <shy828301@...il.com>,
	peterx@...hat.com,
	linux-riscv@...ts.infradead.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Aneesh Kumar K . V" <aneesh.kumar@...nel.org>,
	Rik van Riel <riel@...riel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Mike Rapoport <rppt@...nel.org>,
	John Hubbard <jhubbard@...dia.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Michael Ellerman <mpe@...erman.id.au>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Andrew Jones <andrew.jones@...ux.dev>,
	linuxppc-dev@...ts.ozlabs.org,
	Mike Kravetz <mike.kravetz@...cle.com>,
	Muchun Song <muchun.song@...ux.dev>,
	linux-arm-kernel@...ts.infradead.org,
	Jason Gunthorpe <jgg@...dia.com>,
	Christoph Hellwig <hch@...radead.org>,
	Lorenzo Stoakes <lstoakes@...il.com>,
	Matthew Wilcox <willy@...radead.org>
Subject: [PATCH v2 09/13] mm/gup: Cache *pudp in follow_pud_mask()

From: Peter Xu <peterx@...hat.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.

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

diff --git a/mm/gup.c b/mm/gup.c
index b8a80e2bfe08..63845b3ec44f 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 = READ_ONCE(*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