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

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Date:   Thu, 16 Mar 2017 21:52:23 -0400 (EDT)
From:   Jerome Glisse <jglisse@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Balbir Singh <bsingharora@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        David Nellans <dnellans@...dia.com>,
        Evgeny Baskakov <ebaskakov@...dia.com>,
        Mark Hairgrove <mhairgrove@...dia.com>,
        Sherry Cheung <SCheung@...dia.com>,
        Subhash Gutti <sgutti@...dia.com>
Subject: Re: [HMM 07/16] mm/migrate: new memory migration helper for use
 with device memory v4

> On 03/16/2017 05:45 PM, Balbir Singh wrote:
> > On Fri, Mar 17, 2017 at 11:22 AM, John Hubbard <jhubbard@...dia.com> wrote:
> >> On 03/16/2017 04:05 PM, Andrew Morton wrote:
> >>>
> >>> On Thu, 16 Mar 2017 12:05:26 -0400 Jérôme Glisse <jglisse@...hat.com>
> >>> 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

The original intention was for it to be 64bit only, 32bit is a dying
species and before splitting out hmm_ prefix from this code and moving
it to be generic it was behind a 64bit flag.

If latter one someone really care about 32bit we can only move to u64

Cheers,
Jérôme