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

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Message-ID: <6fd73d30-5525-7f00-1e9c-d7bb96ea34a6@suse.com>
Date:   Fri, 4 Sep 2020 09:00:18 +0200
From:   Jürgen Groß <jgross@...e.com>
To:     Roger Pau Monné <roger.pau@...rix.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 03.09.20 18:38, Roger Pau Monné wrote:
> 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?

Yes, I think so.

I've folded that in and now build is fine.


Juergen