linux-kernel - Re: [PATCH] mm: Correct guards for non_swap

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Message-ID: <efa519d1-5f0a-11a6-e143-c83698b7e5b2@arm.com>
Date:   Fri, 6 Mar 2020 09:41:16 +0000
From:   Steven Price <steven.price@....com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH] mm: Correct guards for non_swap_entry()

On 06/03/2020 03:44, Andrew Morton wrote:
> On Thu,  5 Mar 2020 13:05:50 +0000 Steven Price <steven.price@....com> wrote:
> 
>> If CONFIG_DEVICE_PRIVATE is defined, but neither CONFIG_MEMORY_FAILURE nor
>> CONFIG_MIGRATION, then non_swap_entry() will return 0, meaning that the
>> condition (non_swap_entry(entry) && is_device_private_entry(entry)) in
>> zap_pte_range() will never be true even if the entry is a device private
>> one.
>>
>> Equally any other code depending on non_swap_entry() will not function
>> as expected.
> 
> What are the user-visible runtime effects of this change?
> 
> Is a cc:stable needed?
> 

I originally spotted this just by looking at the code, I haven't
actually observed any problems.

Looking a bit more closely it appears that actually this situation
(currently at least) cannot occur:

DEVICE_PRIVATE depends on ZONE_DEVICE
ZONE_DEVICE depends on MEMORY_HOTREMOVE
MEMORY_HOTREMOVE depends on MIGRATION

So there's probably no need to back port.

Steve