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 17:22:51 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>
CC:     <linux-kernel@...r.kernel.org>, <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 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?

thanks
John Hubbard
NVIDIA