* 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)));
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
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.
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.
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