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

peterx
2024-03-21T22:08:24Z

From: Peter Xu <peterx@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 <jthoughton@google.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Peter Xu <peterx@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