pin_user_pages supports NULL pages arguments?

pin_user_pages supports NULL pages arguments?

[Minchan Kim]

 * pin_user_pages() - pin user pages in memory for use by other devices
 < snip >
 * @pages:      array that receives pointers to the pages pinned.
 *              Should be at least nr_pages long. Or NULL, if caller
 *              only intends to ensure the pages are faulted in.

pin_user_pages(,, pages = NULL, );
  gup_flags |= FOLL_PIN
  __get_user_pages_locked
    __get_user_pages
      ..
      VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));

Re: pin_user_pages supports NULL pages arguments?

[John Hubbard]

On 2/10/22 11:17, Minchan Kim wrote:
>   * pin_user_pages() - pin user pages in memory for use by other devices
>   < snip >
>   * @pages:      array that receives pointers to the pages pinned.
>   *              Should be at least nr_pages long. Or NULL, if caller
>   *              only intends to ensure the pages are faulted in.
> 
> pin_user_pages(,, pages = NULL, );
>    gup_flags |= FOLL_PIN
>    __get_user_pages_locked
>      __get_user_pages
>        ..
>        VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));

Only FOLL_GET or FOLL_PIN are supposed to fill in the **pages array. So
if a caller passes a null **pages arg, then that caller must not also
set FOLL_GET or FOLL_PIN. That's what the VM_BUG_ON() is expressing.

Perhaps that should be part of the documentation. It sort of is already,
for get_user_pages*().

thanks,
-- 
John Hubbard
NVIDIA

Re: pin_user_pages supports NULL pages arguments?

[Minchan Kim]

On Thu, Feb 10, 2022 at 11:20:31AM -0800, John Hubbard wrote:
> On 2/10/22 11:17, Minchan Kim wrote:
> >   * pin_user_pages() - pin user pages in memory for use by other devices
> >   < snip >
> >   * @pages:      array that receives pointers to the pages pinned.
> >   *              Should be at least nr_pages long. Or NULL, if caller
> >   *              only intends to ensure the pages are faulted in.
> > 
> > pin_user_pages(,, pages = NULL, );
> >    gup_flags |= FOLL_PIN
> >    __get_user_pages_locked
> >      __get_user_pages
> >        ..
> >        VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));
> 
> Only FOLL_GET or FOLL_PIN are supposed to fill in the **pages array. So
> if a caller passes a null **pages arg, then that caller must not also
> set FOLL_GET or FOLL_PIN. That's what the VM_BUG_ON() is expressing.

Yub, but pin_user_pages adds FOLL_PIN unconditinally and the comments
says it supports NUU pages argument. Isn't it conflict?

> 
> Perhaps that should be part of the documentation. It sort of is already,
> for get_user_pages*().

I expected it was just copied from get_user_pages.

Re: pin_user_pages supports NULL pages arguments?

[John Hubbard]

On 2/10/22 11:29, Minchan Kim wrote:
> On Thu, Feb 10, 2022 at 11:20:31AM -0800, John Hubbard wrote:
>> On 2/10/22 11:17, Minchan Kim wrote:
>>>    * pin_user_pages() - pin user pages in memory for use by other devices
>>>    < snip >
>>>    * @pages:      array that receives pointers to the pages pinned.
>>>    *              Should be at least nr_pages long. Or NULL, if caller
>>>    *              only intends to ensure the pages are faulted in.
>>>
>>> pin_user_pages(,, pages = NULL, );
>>>     gup_flags |= FOLL_PIN
>>>     __get_user_pages_locked
>>>       __get_user_pages
>>>         ..
>>>         VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));
>>
>> Only FOLL_GET or FOLL_PIN are supposed to fill in the **pages array. So
>> if a caller passes a null **pages arg, then that caller must not also
>> set FOLL_GET or FOLL_PIN. That's what the VM_BUG_ON() is expressing.
> 
> Yub, but pin_user_pages adds FOLL_PIN unconditinally and the comments
> says it supports NUU pages argument. Isn't it conflict?
>

Oh right, that is a conflict. The documentation should *not* say that a
NULL **pages arg is supported. Because the whole point of the FOLL_PIN
APIs is to actually pin struct pages. The NULL cases are only useful for
get_user_pages*().

So removing that last sentence is appropriate, plus also looking around
for similar documentation claims, including in pin_user_pages.rst. I
don't see anything from a very quick scan, though.

Sending out a fix will also trigger the observation that both the
kerneldoc headers mm/gup.c, and the writings in pin_user_pages.rst,
could use some updating. However, it is also true that this can be
reasonably treated as a documentation bug fix, and therefore allowed to
be limited to just this change.

Were you going to send out a formal patch? If not, I can include it in
an upcoming gup series, with your Reported-by tag. Up to you.


>>
>> Perhaps that should be part of the documentation. It sort of is already,
>> for get_user_pages*().
> 
> I expected it was just copied from get_user_pages.

Yes it was.

thanks,
-- 
John Hubbard
NVIDIA

Re: pin_user_pages supports NULL pages arguments?

[Minchan Kim]

On Thu, Feb 10, 2022 at 12:20:47PM -0800, John Hubbard wrote:
> On 2/10/22 11:29, Minchan Kim wrote:
> > On Thu, Feb 10, 2022 at 11:20:31AM -0800, John Hubbard wrote:
> > > On 2/10/22 11:17, Minchan Kim wrote:
> > > >    * pin_user_pages() - pin user pages in memory for use by other devices
> > > >    < snip >
> > > >    * @pages:      array that receives pointers to the pages pinned.
> > > >    *              Should be at least nr_pages long. Or NULL, if caller
> > > >    *              only intends to ensure the pages are faulted in.
> > > > 
> > > > pin_user_pages(,, pages = NULL, );
> > > >     gup_flags |= FOLL_PIN
> > > >     __get_user_pages_locked
> > > >       __get_user_pages
> > > >         ..
> > > >         VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));
> > > 
> > > Only FOLL_GET or FOLL_PIN are supposed to fill in the **pages array. So
> > > if a caller passes a null **pages arg, then that caller must not also
> > > set FOLL_GET or FOLL_PIN. That's what the VM_BUG_ON() is expressing.
> > 
> > Yub, but pin_user_pages adds FOLL_PIN unconditinally and the comments
> > says it supports NUU pages argument. Isn't it conflict?
> > 
> 
> Oh right, that is a conflict. The documentation should *not* say that a
> NULL **pages arg is supported. Because the whole point of the FOLL_PIN
> APIs is to actually pin struct pages. The NULL cases are only useful for
> get_user_pages*().
> 
> So removing that last sentence is appropriate, plus also looking around
> for similar documentation claims, including in pin_user_pages.rst. I
> don't see anything from a very quick scan, though.
> 
> Sending out a fix will also trigger the observation that both the
> kerneldoc headers mm/gup.c, and the writings in pin_user_pages.rst,
> could use some updating. However, it is also true that this can be
> reasonably treated as a documentation bug fix, and therefore allowed to
> be limited to just this change.
> 
> Were you going to send out a formal patch? If not, I can include it in
> an upcoming gup series, with your Reported-by tag. Up to you.

Yes, please.

Thank you, John.