linux-kernel - Re: [HMM 12/15] mm/migrate: new memory migration helper for use with device memory v4

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Date:   Fri, 30 Jun 2017 20:57:50 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Evgeny Baskakov <ebaskakov@...dia.com>
Cc:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.com>,
        David Nellans <dnellans@...dia.com>,
        Mark Hairgrove <mhairgrove@...dia.com>,
        Sherry Cheung <SCheung@...dia.com>,
        Subhash Gutti <sgutti@...dia.com>
Subject: Re: [HMM 12/15] mm/migrate: new memory migration helper for use with
 device memory v4

On Fri, Jun 30, 2017 at 04:19:25PM -0700, Evgeny Baskakov wrote:
> Hi Jerome,
> 
> It seems that the kernel can pass 0 in src_pfns for pages that it cannot
> migrate (i.e. the kernel knows that they cannot migrate prior to calling
> alloc_and_copy).
> 
> So, a zero in src_pfns can mean either "the page is not allocated yet" or
> "the page cannot migrate".
> 
> Can migrate_vma set the MIGRATE_PFN_MIGRATE flag for not allocated pages? On
> the driver side it is difficult to differentiate between the cases.

So this is what is happening in v24. For thing that can not be migrated you
get 0 and for things that are not allocated you get MIGRATE_PFN_MIGRATE like
the updated comments in migrate.h explain.

Cheers,
Jérôme