[PATCH 02/13] mm/gup: Cache p4d in follow_p4d_mask()

[peterx]

From: Peter Xu <pet...@redhat.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 <pet...@redhat.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