Re: [HMM 07/16] mm/migrate: new memory migration helper for use with device memory v4

[John Hubbard]

On 03/16/2017 05:45 PM, Balbir Singh wrote:
> On Fri, Mar 17, 2017 at 11:22 AM, John Hubbard <jhubbard@xxxxxxxxxx> wrote:
> > On 03/16/2017 04:05 PM, Andrew Morton wrote:
> > > On Thu, 16 Mar 2017 12:05:26 -0400 JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> > > wrote:
> > >
> > > > +static inline struct page *migrate_pfn_to_page(unsigned long mpfn)
> > > > +{
> > > > +       if (!(mpfn & MIGRATE_PFN_VALID))
> > > > +               return NULL;
> > > > +       return pfn_to_page(mpfn & MIGRATE_PFN_MASK);
> > > > +}
> > >
> > >
> > > i386 allnoconfig:
> > >
> > > In file included from mm/page_alloc.c:61:
> > > ./include/linux/migrate.h: In function 'migrate_pfn_to_page':
> > > ./include/linux/migrate.h:139: warning: left shift count >= width of type
> > > ./include/linux/migrate.h:141: warning: left shift count >= width of type
> > > ./include/linux/migrate.h: In function 'migrate_pfn_size':
> > > ./include/linux/migrate.h:146: warning: left shift count >= width of type
> >
> > It seems clear that this was never meant to work with < 64-bit pfns:
> >
> > // migrate.h excerpt:
> > #define MIGRATE_PFN_VALID       (1UL << (BITS_PER_LONG_LONG - 1))
> > #define MIGRATE_PFN_MIGRATE     (1UL << (BITS_PER_LONG_LONG - 2))
> > #define MIGRATE_PFN_HUGE        (1UL << (BITS_PER_LONG_LONG - 3))
> > #define MIGRATE_PFN_LOCKED      (1UL << (BITS_PER_LONG_LONG - 4))
> > #define MIGRATE_PFN_WRITE       (1UL << (BITS_PER_LONG_LONG - 5))
> > #define MIGRATE_PFN_DEVICE      (1UL << (BITS_PER_LONG_LONG - 6))
> > #define MIGRATE_PFN_ERROR       (1UL << (BITS_PER_LONG_LONG - 7))
> > #define MIGRATE_PFN_MASK        ((1UL << (BITS_PER_LONG_LONG - PAGE_SHIFT))
> > - 1)
> >
> > ...obviously, there is not enough room for these flags, in a 32-bit pfn.
> >
> > So, given the current HMM design, I think we are going to have to provide a
> > 32-bit version of these routines (migrate_pfn_to_page, and related) that is
> > a no-op, right?
>
> Or make the HMM Kconfig feature 64BIT only by making it depend on 64BIT?

Yes, that was my first reaction too, but these particular routines are aspiring to be generic 
routines--in fact, you have had an influence there, because these might possibly help with NUMA 
migrations. :)

So it would look odd to see this:

#ifdef CONFIG_HMM
int migrate_vma(const struct migrate_vma_ops *ops,
		struct vm_area_struct *vma,
		unsigned long mentries,
		unsigned long start,
		unsigned long end,
		unsigned long *src,
		unsigned long *dst,
		void *private)
{
   //...implementation
#endif

...because migrate_vma() does not sound HMM-specific, and it is, after all, in migrate.h and 
migrate.c. We probably want this a more generic approach (not sure if I've picked exactly the right 
token to #ifdef on, but it's close):

#ifdef CONFIG_64BIT
int migrate_vma(const struct migrate_vma_ops *ops,
		struct vm_area_struct *vma,
		unsigned long mentries,
		unsigned long start,
		unsigned long end,
		unsigned long *src,
		unsigned long *dst,
		void *private)
{
   /* ... full implementation */
}

#else
int migrate_vma(const struct migrate_vma_ops *ops,
		struct vm_area_struct *vma,
		unsigned long mentries,
		unsigned long start,
		unsigned long end,
		unsigned long *src,
		unsigned long *dst,
		void *private)
{
   return -EINVAL; /* or something more appropriate */
}
#endif

thanks
John Hubbard
NVIDIA

> Balbir Singh