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 17:57:35 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Balbir Singh <bsingharora@...il.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        "linux-kernel@...r.kernel.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

>
> Balbir Singh
>