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

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Date:   Fri, 17 Mar 2017 11:45:19 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     John Hubbard <jhubbard@...dia.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 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?


Balbir Singh