linux-kernel - Re: [PATCH v5 3/3] xen: add helpers to allocate unpopulated memory

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Date:   Thu, 3 Sep 2020 17:30:07 +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 v5 3/3] xen: add helpers to allocate unpopulated memory

On 01.09.20 10:33, 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.
> 
> Note the driver is currently not enabled on Arm platforms because it
> would interfere with the identity mapping required on some platforms.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>

Sorry, I just got a build error for x86 32-bit build:

WARNING: unmet direct dependencies detected for ZONE_DEVICE
   Depends on [n]: MEMORY_HOTPLUG [=n] && MEMORY_HOTREMOVE [=n] && 
SPARSEMEM_VMEMMAP [=n] && ARCH_HAS_PTE_DEVMAP [=n]
   Selected by [y]:
   - XEN_UNPOPULATED_ALLOC [=y] && XEN [=y] && X86 [=y]
   GEN     Makefile
   CC      kernel/bounds.s
   CALL    /home/gross/korg/src/scripts/atomic/check-atomics.sh
   UPD     include/generated/bounds.h
   CC      arch/x86/kernel/asm-offsets.s
In file included from /home/gross/korg/src/include/linux/mmzone.h:19:0,
                  from /home/gross/korg/src/include/linux/gfp.h:6,
                  from /home/gross/korg/src/include/linux/slab.h:15,
                  from /home/gross/korg/src/include/linux/crypto.h:19,
                  from /home/gross/korg/src/arch/x86/kernel/asm-offsets.c:9:
/home/gross/korg/src/include/linux/page-flags-layout.h:95:2: error: 
#error "Not enough bits in page flags"
  #error "Not enough bits in page flags"
   ^~~~~
make[2]: *** [/home/gross/korg/src/scripts/Makefile.build:114: 
arch/x86/kernel/asm-offsets.s] Error 1
make[1]: *** [/home/gross/korg/src/Makefile:1175: prepare0] Error 2
make[1]: Leaving directory '/home/gross/korg/x8632'
make: *** [Makefile:185: __sub-make] Error 2


Juergen