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

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Message-ID: <20200903163837.GM753@Air-de-Roger>
Date:   Thu, 3 Sep 2020 18:38:37 +0200
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Jürgen Groß <jgross@...e.com>
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 v5 3/3] xen: add helpers to allocate unpopulated memory

On Thu, Sep 03, 2020 at 05:30:07PM +0200, Jürgen Groß wrote:
> 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

Sorry for this. I've tested a 32bit build but I think it was before
the last Kconfig changes. I'm a little unsure how to solve this, as
ZONE_DEVICE doesn't select the required options for it to run, but
rather depends on them to be available.

You can trigger something similar on x86-64 by doing:

$ make ARCH=x86_64 xen.config
Using .config as base
Merging ./kernel/configs/xen.config
Merging ./arch/x86/configs/xen.config
#
# merged configuration written to .config (needs make)
#
scripts/kconfig/conf  --olddefconfig Kconfig

WARNING: unmet direct dependencies detected for ZONE_DEVICE
  Depends on [n]: MEMORY_HOTPLUG [=y] && MEMORY_HOTREMOVE [=n] && SPARSEMEM_VMEMMAP [=y] && ARCH_HAS_PTE_DEVMAP [=y]
  Selected by [y]:
  - XEN_UNPOPULATED_ALLOC [=y] && XEN [=y] && X86_64 [=y]
#
# configuration written to .config
#

I think the only solution is to have XEN_UNPOPULATED_ALLOC depend on
ZONE_DEVICE rather than select it?

Thanks, Roger.