linux-kernel - Re: [PATCH v4 2/2] xen: add helpers to allocate unpopulated memory

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Message-ID: <a907ff4a-c887-7d02-1d45-140d7749afa4@suse.com>
Date:   Fri, 14 Aug 2020 12:27:32 +0200
From:   Jürgen Groß <jgross@...e.com>
To:     Roger Pau Monné <roger.pau@...rix.com>,
        Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Dan Carpenter <dan.carpenter@...cle.com>, Wei Liu <wl@....org>,
        Yan Yankovskyi <yyankovskyi@...il.com>,
        dri-devel@...ts.freedesktop.org, xen-devel@...ts.xenproject.org,
        linux-mm@...ck.org, David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v4 2/2] xen: add helpers to allocate unpopulated memory

On 14.08.20 11:56, Roger Pau Monné wrote:
> On Fri, Aug 14, 2020 at 08:29:20AM +0100, Christoph Hellwig wrote:
>> On Thu, Aug 13, 2020 at 09:54:20AM +0200, Roger Pau Monn?? wrote:
>>> On Thu, Aug 13, 2020 at 08:33:37AM +0100, Christoph Hellwig wrote:
>>>> On Tue, Aug 11, 2020 at 11:44:47AM +0200, Roger Pau Monne wrote:
>>>>> If enabled (because ZONE_DEVICE is supported) the usage of the new
>>>>> functionality untangles Xen balloon and RAM hotplug from the usage of
>>>>> unpopulated physical memory ranges to map foreign pages, which is the
>>>>> correct thing to do in order to avoid mappings of foreign pages depend
>>>>> on memory hotplug.
>>>>
>>>> So please just select ZONE_DEVICE if this is so much better rather
>>>> than maintaining two variants.
>>>
>>> We still need to other variant for Arm at least, so both need to be
>>> maintained anyway, even if we force ZONE_DEVICE on x86.
>>
>> Well, it still really helps reproducability if you stick to one
>> implementation of x86.
>>
>> The alternative would be an explicit config option to opt into it,
>> but just getting a different implementation based on a random
>> kernel option is strange.
> 
> Would adding something like the chunk below to the patch be OK?
> 
> ---8<---
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index 018020b91baa..5f321a1319e6 100644
> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -328,7 +328,14 @@ config XEN_FRONT_PGDIR_SHBUF
>   	tristate
>   
>   config XEN_UNPOPULATED_ALLOC
> -	bool
> -	default y if ZONE_DEVICE && !ARM && !ARM64
> +	bool "Use unpopulated memory ranges for guest mappings"
> +	depends on X86
> +	select ZONE_DEVICE
> +	default y

I'd rather use "default XEN_BACKEND" here, as mappings of other guest's
memory is rarely used for non-backend guests.


Juergen