[PATCH v3 08/12] mm/gup: Cache *pudp in follow_pud_mask()

[peterx]

From: Peter Xu <pet...@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.

Acked-by: James Houghton <jthough...@google.com>
Reviewed-by: Jason Gunthorpe <j...@nvidia.com>
Signed-off-by: Peter Xu <pet...@redhat.com>
---
 mm/gup.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 943671736080..a338944e4425 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,
-- 
2.44.0