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[RFC,03/13] mm/gup: Check p4d presence before going on

Message ID 20240306104147.193052-4-peterx@redhat.com
State New
Headers show
Series mm/treewide: Remove pXd_huge() API | expand

Commit Message

Peter Xu March 6, 2024, 10:41 a.m. UTC 
From: Peter Xu <peterx@redhat.com>

Currently there should have no p4d swap entries so it may not matter much,
however this may help us to rule out swap entries in pXd_huge() API, which
will include p4d_huge().  The p4d_present() checks make it 100% clear that
we won't rely on p4d_huge() for swap entries.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/gup.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Jason Gunthorpe March 7, 2024, 8:08 p.m. UTC | #1 
On Wed, Mar 06, 2024 at 06:41:37PM +0800, peterx@redhat.com wrote:
> From: Peter Xu <peterx@redhat.com>
> 
> Currently there should have no p4d swap entries so it may not matter much,
> however this may help us to rule out swap entries in pXd_huge() API, which
> will include p4d_huge().  The p4d_present() checks make it 100% clear that
> we won't rely on p4d_huge() for swap entries.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  mm/gup.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

>  	p4dp = p4d_offset(pgdp, address);
>  	p4d = READ_ONCE(*p4dp);
> -	if (p4d_none(p4d))
> +	if (p4d_none(p4d) || !p4d_present(p4d))
>  		return no_page_table(vma, flags);

Like the other place I think this makes more sense as

    if (!p4_present(p4d))
  		return no_page_table(vma, flags);

Since none can never be present.

IOW if the following code doesn't decode swap entries then it should
be guarded with a chceck on present not none..

Jason
diff mbox series

Patch

diff --git a/mm/gup.c b/mm/gup.c
index 69a777f4fc5c..f87a7d90f2ae 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -776,7 +776,7 @@  static struct page *follow_p4d_mask(struct vm_area_struct *vma,
 
 	p4dp = p4d_offset(pgdp, address);
 	p4d = READ_ONCE(*p4dp);
-	if (p4d_none(p4d))
+	if (p4d_none(p4d) || !p4d_present(p4d))
 		return no_page_table(vma, flags);
 	BUILD_BUG_ON(p4d_huge(p4d));
 	if (unlikely(p4d_bad(p4d)))
@@ -3069,7 +3069,7 @@  static int gup_p4d_range(pgd_t *pgdp, pgd_t pgd, unsigned long addr, unsigned lo
 		p4d_t p4d = READ_ONCE(*p4dp);
 
 		next = p4d_addr_end(addr, end);
-		if (p4d_none(p4d))
+		if (p4d_none(p4d) || !p4d_present(p4d))
 			return 0;
 		BUILD_BUG_ON(p4d_huge(p4d));
 		if (unlikely(is_hugepd(__hugepd(p4d_val(p4d))))) {