mm/gup: remove unneeded checking in follow_page_pte()

Patch series "mm/gup: Minor fix, cleanup and improvements", v4.

These were made from code inspection in mm/gup.c.


This patch (of 3):

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().

Link: https://lkml.kernel.org/r/20250410035717.473207-1-bhe@redhat.com
Link: https://lkml.kernel.org/r/20250410035717.473207-3-bhe@redhat.com
Signed-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Yanjun.Zhu <yanjun.zhu@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files 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;