Re: [PATCH v5 1/6] mm/gup: remove unused vmas parameter from get_user_pages()

[David Hildenbrand]

On 15.05.23 21:07, Sean Christopherson wrote:
> On Sun, May 14, 2023, Lorenzo Stoakes wrote:
> > No invocation of get_user_pages() use the vmas parameter, so remove it.
> >
> > The GUP API is confusing and caveated. Recent changes have done much to
> > improve that, however there is more we can do. Exporting vmas is a prime
> > target as the caller has to be extremely careful to preclude their use
> > after the mmap_lock has expired or otherwise be left with dangling
> > pointers.
> >
> > Removing the vmas parameter focuses the GUP functions upon their primary
> > purpose - pinning (and outputting) pages as well as performing the actions
> > implied by the input flags.
> >
> > This is part of a patch series aiming to remove the vmas parameter
> > altogether.
> >
> > Suggested-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > Acked-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> > Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > Acked-by: Christian K�nig <christian.koenig@xxxxxxx> (for radeon parts)
> > Acked-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> > Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> > ---
> >   arch/x86/kernel/cpu/sgx/ioctl.c     | 2 +-
> >   drivers/gpu/drm/radeon/radeon_ttm.c | 2 +-
> >   drivers/misc/sgi-gru/grufault.c     | 2 +-
> >   include/linux/mm.h                  | 3 +--
> >   mm/gup.c                            | 9 +++------
> >   mm/gup_test.c                       | 5 ++---
> >   virt/kvm/kvm_main.c                 | 2 +-
> >   7 files changed, 10 insertions(+), 15 deletions(-)
>
> Acked-by: Sean Christopherson <seanjc@xxxxxxxxxx> (KVM)
>
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index cb5c13eee193..eaa5bb8dbadc 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -2477,7 +2477,7 @@ static inline int check_user_page_hwpoison(unsigned long addr)
> >   {
> >   	int rc, flags = FOLL_HWPOISON | FOLL_WRITE;
> >   
> > -	rc = get_user_pages(addr, 1, flags, NULL, NULL);
> > +	rc = get_user_pages(addr, 1, flags, NULL);
> >   	return rc == -EHWPOISON;
>
> Unrelated to this patch, I think there's a pre-existing bug here.  If gup() returns
> a valid page, KVM will leak the refcount and unintentionally pin the page.  That's

When passing NULL as "pages" to get_user_pages(), 
__get_user_pages_locked() won't set FOLL_GET. As FOLL_PIN is also not 
set, we won't be messing with the mapcount of the page.

So even if get_user_pages() returns "1", we should be fine.


Or am I misunderstanding your concern? At least hva_to_pfn_slow() most 
certainly didn't return "1" if we end up calling 
check_user_page_hwpoison(), so nothing would have been pinned there as well.

--
Thanks,

David / dhildenb