linux-kernel - Re: [PATCH] mm/migrate: properly preserve write attribute in special migrate entry

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Message-Id: <20180403153046.88cae4ab18646e8e23a648ce@linux-foundation.org>
Date:   Tue, 3 Apr 2018 15:30:46 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     jglisse@...hat.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Ralph Campbell <rcampbell@...dia.com>
Subject: Re: [PATCH] mm/migrate: properly preserve write attribute in
 special migrate entry

On Sun,  1 Apr 2018 22:35:06 -0400 jglisse@...hat.com wrote:

> From: Ralph Campbell <rcampbell@...dia.com>
> 
> Use of pte_write(pte) is only valid for present pte, the common code
> which set the migration entry can be reach for both valid present
> pte and special swap entry (for device memory). Fix the code to use
> the mpfn value which properly handle both cases.
> 
> On x86 this did not have any bad side effect because pte write bit
> is below PAGE_BIT_GLOBAL and thus special swap entry have it set to
> 0 which in turn means we were always creating read only special
> migration entry.

Does this mean that the patch only affects behaviour of non-x86 systems?

> So once migration did finish we always write protected the CPU page
> table entry (moreover this is only an issue when migrating from device
> memory to system memory). End effect is that CPU write access would
> fault again and restore write permission.

That sounds a bit serious.  Was a -stable backport considered?