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

From: "Jürgen Groß" <jgross@suse.com>
To: Roger Pau Monne <roger.pau@citrix.com>, linux-kernel@vger.kernel.org
Cc: Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>,
	David Airlie <airlied@linux.ie>,
	Yan Yankovskyi <yyankovskyi@gmail.com>,
	David Hildenbrand <david@redhat.com>,
	dri-devel@lists.freedesktop.org, Michal Hocko <mhocko@kernel.org>,
	linux-mm@kvack.org, Daniel Vetter <daniel@ffwll.ch>,
	xen-devel@lists.xenproject.org,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Dan Carpenter <dan.carpenter@oracle.com>
Subject: Re: [PATCH v3 4/4] xen: add helpers to allocate unpopulated memory
Date: Mon, 27 Jul 2020 17:09:07 +0200	[thread overview]
Message-ID: <06e21488-25a6-1c9f-366b-3c2ab63e4632@suse.com> (raw)
In-Reply-To: <20200727091342.52325-5-roger.pau@citrix.com>

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@citrix.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@suse.com>

Juergen