[PATCH v4 2/4] mm/gup: remove unneeded checking in follow_page_pte()

Baoquan He
2025-04-10T03:57:39Z

In __get_user_pages(), it will traverse page table and take a reference
to the page the given user address corresponds to if GUP_GET or GUP_PIN
is set. However, it's not supported both GUP_GET and GUP_PIN are set.
Even though this check need be done, it should be done earlier, but not
doing it till entering into follow_page_pte() and failed.

Furthermore, this checking has been done in is_valid_gup_args() and all
external users of __get_user_pages() will call is_valid_gup_args() to
catch the illegal setting. We don't need to worry about internal users
of __get_user_pages() because the gup_flags are set by MM code correctly.

Here remove the checking in follow_page_pte(), and add VM_WARN_ON_ONCE()
to catch the possible exceptional setting just in case.

And also change the VM_BUG_ON to VM_WARN_ON_ONCE() for checking
(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN))) because the checking
has been done in is_valid_gup_args() for external users of
__get_user_pages().

Signed-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Acked-by: David Hildenbrand <david@redhat.com>
---
mm/gup.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 84461d384ae2..eb668da933e1 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -844,11 +844,6 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
pte_t *ptep, pte;
int ret;

- /* FOLL_GET and FOLL_PIN are mutually exclusive. */
- if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
- (FOLL_PIN | FOLL_GET)))
- return ERR_PTR(-EINVAL);
-
ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
if (!ptep)
return no_page_table(vma, flags, address);
@@ -1432,7 +1427,11 @@ static long __get_user_pages(struct mm_struct *mm,

start = untagged_addr_remote(mm, start);

- VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));
+ VM_WARN_ON_ONCE(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));
+
+ /* FOLL_GET and FOLL_PIN are mutually exclusive. */
+ VM_WARN_ON_ONCE((gup_flags & (FOLL_PIN | FOLL_GET)) ==
+ (FOLL_PIN | FOLL_GET));

do {
struct page *page;
--
2.41.0