mm/gup: remove unneeded checking in follow_page_pte()

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>