Re: [PATCH 4/4] mm/gup: adapt get_user_page_vma_remote() to never return NULL

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Mon, Oct 02, 2023 at 01:08:41PM +0200, David Hildenbrand wrote:
> On 01.10.23 18:00, Lorenzo Stoakes wrote:
> > get_user_pages_remote() will never return 0 except in the case of
> > FOLL_NOWAIT being specified, which we explicitly disallow.
> >
> > This simplifies error handling for the caller and avoids the awkwardness of
> > dealing with both errors and failing to pin. Failing to pin here is an
> > error.
> >
> > Suggested-by: Arnd Bergmann <arnd@arndb.de>
> > Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> > ---
> >   arch/arm64/kernel/mte.c |  4 ++--
> >   include/linux/mm.h      | 16 +++++++++++++---
> >   kernel/events/uprobes.c |  4 ++--
> >   mm/memory.c             |  3 +--
> >   4 files changed, 18 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> > index 4edecaac8f91..8878b392df58 100644
> > --- a/arch/arm64/kernel/mte.c
> > +++ b/arch/arm64/kernel/mte.c
> > @@ -411,8 +411,8 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
> >   		struct page *page = get_user_page_vma_remote(mm, addr,
> >   							     gup_flags, &vma);
> > -		if (IS_ERR_OR_NULL(page)) {
> > -			err = page == NULL ? -EIO : PTR_ERR(page);
> > +		if (IS_ERR(page)) {
> > +			err = PTR_ERR(page);
> >   			break;
> >   		}
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 7b89f7bd420d..da9631683d38 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -2425,6 +2425,7 @@ long pin_user_pages_remote(struct mm_struct *mm,
> >   			   unsigned int gup_flags, struct page **pages,
> >   			   int *locked);
> > +/* Either retrieve a single VMA and page, or an error. */
> >   static inline struct page *get_user_page_vma_remote(struct mm_struct *mm,
> >   						    unsigned long addr,
> >   						    int gup_flags,
> > @@ -2432,12 +2433,21 @@ static inline struct page *get_user_page_vma_remote(struct mm_struct *mm,
> >   {
> >   	struct page *page;
> >   	struct vm_area_struct *vma;
> > -	int got = get_user_pages_remote(mm, addr, 1, gup_flags, &page, NULL);
> > +	int got;
> > +
> > +	if (unlikely(gup_flags & FOLL_NOWAIT))
> > +		return ERR_PTR(-EINVAL);
> > +
>
> Do we have any callers or do we want to make that official (document it) and
> use WARN_ON_ONCE() instead?

We have no callers who want to do that, will WARN_ON_ONCE() and update
comment in v2.

>
> > +	got = get_user_pages_remote(mm, addr, 1, gup_flags, &page, NULL);
> >   	if (got < 0)
> >   		return ERR_PTR(got);
> > -	if (got == 0)
> > -		return NULL;
> > +
> > +	/*
> > +	 * get_user_pages_remote() is guaranteed to not return 0 for
> > +	 * non-FOLL_NOWAIT contexts, so this should never happen.
> > +	 */
> > +	VM_WARN_ON(got == 0);
>
> You should probably just drop that. Not worth the comment + code and its
> better checked inside get_user_pages_remote().
>
> Ideally, just document that behavior for get_user_pages_remote() "Will never
> return 0 without FOLL_NOWAIT."

Well you'd need to put it at all the other callers of
__get_user_pages_locked() too :) so I think probably not worth doing that,
at least in this patch series.

In any case, we have explicitly added this check there to ensure that's not
possible so I think we're good, will just strip in v2.

>
> --
> Cheers,
>
> David / dhildenb
>