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 14:42:05 +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

>> 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. :)
>

Yes, I still stick to them being generic, but I'd be OK if they worked
just for 64 bit systems.
Having said that even the 64 bit works version work for upto physical
sizes of 64 - PAGE_SHIFT
which is a little limiting I think.

One option is to make pfn's unsigned long long and do 32 and 64 bit computations
separately

Option 2, could be something like you said

a. Define a __weak migrate_vma to return -EINVAL
b. In a 64BIT only file define migrate_vma

Option 3

Something totally different

If we care to support 32 bit we go with 1, else option 2 is a good
starting point. There might
be other ways of doing option 2, like you've suggested

Balbir