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Message-ID: <20240103091423.400294-10-peterx@redhat.com>
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
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