linux-kernel - [PATCH RFC 02/13] mm/gup: Cache p4d in follow_p4d

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Message-ID: <20240306104147.193052-3-peterx@redhat.com>
Date: Wed,  6 Mar 2024 18:41:36 +0800
From: peterx@...hat.com
To: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Cc: linuxppc-dev@...ts.ozlabs.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Muchun Song <muchun.song@...ux.dev>,
	Jason Gunthorpe <jgg@...dia.com>,
	peterx@...hat.com,
	Matthew Wilcox <willy@...radead.org>,
	Mike Rapoport <rppt@...nel.org>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	x86@...nel.org,
	sparclinux@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: [PATCH RFC 02/13] mm/gup: Cache p4d in follow_p4d_mask()

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

Add a variable to cache p4d in follow_p4d_mask().  It's a good practise to
make sure all the following checks will have a consistent view of the entry.

Signed-off-by: Peter Xu <peterx@...hat.com>
---
 mm/gup.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index df83182ec72d..69a777f4fc5c 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -772,16 +772,17 @@ static struct page *follow_p4d_mask(struct vm_area_struct *vma,
 				    unsigned int flags,
 				    struct follow_page_context *ctx)
 {
-	p4d_t *p4d;
+	p4d_t *p4dp, p4d;
 
-	p4d = p4d_offset(pgdp, address);
-	if (p4d_none(*p4d))
+	p4dp = p4d_offset(pgdp, address);
+	p4d = READ_ONCE(*p4dp);
+	if (p4d_none(p4d))
 		return no_page_table(vma, flags);
-	BUILD_BUG_ON(p4d_huge(*p4d));
-	if (unlikely(p4d_bad(*p4d)))
+	BUILD_BUG_ON(p4d_huge(p4d));
+	if (unlikely(p4d_bad(p4d)))
 		return no_page_table(vma, flags);
 
-	return follow_pud_mask(vma, address, p4d, flags, ctx);
+	return follow_pud_mask(vma, address, p4dp, flags, ctx);
 }
 
 /**
-- 
2.44.0