Re: [PATCH v3 2/3] mm/gup: remove unneeded checking in follow_page_pte()

[David Hildenbrand]

On 08.04.25 11:04, Oscar Salvador wrote:
> On Mon, Apr 07, 2025 at 11:03:05AM +0800, Baoquan He wrote:
> > 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().
>
> I will also note that you changed the flags check to VM_WARN_ON_ONCE.
> I guess this is fine as we have the WARN_ON_ONCE version in
> is_valid_gup_args().

Yes, that was my reasoning when I proposed it.

> > Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
>
> LGTM,
>
> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb