diff mbox series

[v2,09/13] mm/gup: Cache *pudp in follow_pud_mask()

Message ID 20240103091423.400294-10-peterx@redhat.com
State New
Headers
Series mm/gup: Unify hugetlb, part 2 |

Commit Message

Peter Xu Jan. 3, 2024, 9:14 a.m. UTC 
  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>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/gup.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

Comments

Jason Gunthorpe Jan. 15, 2024, 6:41 p.m. UTC | #1 
On Wed, Jan 03, 2024 at 05:14:19PM +0800, peterx@redhat.com wrote:
> 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>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  mm/gup.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)

I think we have several more case like this, and I ceratinly agree
code should not access a READ_ONCE variable more than once :(

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason
diff mbox series

Patch

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,