linux-kernel - Re: [PATCH v3 4/4] xen: add helpers to allocate unpopulated memory

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Date:   Mon, 27 Jul 2020 17:09:07 +0200
From:   Jürgen Groß <jgross@...e.com>
To:     Roger Pau Monne <roger.pau@...rix.com>,
        linux-kernel@...r.kernel.org
Cc:     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 v3 4/4] xen: add helpers to allocate unpopulated memory

On 27.07.20 11:13, Roger Pau Monne wrote:
> To be used in order to create foreign mappings. This is based on the
> ZONE_DEVICE facility which is used by persistent memory devices in
> order to create struct pages and kernel virtual mappings for the IOMEM
> areas of such devices. Note that on kernels without support for
> ZONE_DEVICE Xen will fallback to use ballooned pages in order to
> create foreign mappings.
> 
> The newly added helpers use the same parameters as the existing
> {alloc/free}_xenballooned_pages functions, which allows for in-place
> replacement of the callers. Once a memory region has been added to be
> used as scratch mapping space it will no longer be released, and pages
> returned are kept in a linked list. This allows to have a buffer of
> pages and prevents resorting to frequent additions and removals of
> regions.
> 
> 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.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>
> ---
> I've not added a new memory_type type and just used
> MEMORY_DEVICE_DEVDAX which seems to be what we want for such memory
> regions. I'm unsure whether abusing this type is fine, or if I should
> instead add a specific type, maybe MEMORY_DEVICE_GENERIC? I don't
> think we should be using a specific Xen type at all.

What about introducing MEMORY_DEVICE_VIRT? The comment for
MEMORY_DEVICE_DEVDAX doesn't really fit, as there is no character device
involved.

In the end its the memory management maintainers to decide that.

Other than that you can have my

Reviewed-by: Juergen Gross <jgross@...e.com>


Juergen