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

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Message-ID: <41905296-c157-938b-4953-19af3435c085@suse.com>
Date:   Fri, 4 Sep 2020 14:40:26 +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 04.09.20 10:42, Roger Pau Monné wrote:
> On Fri, Sep 04, 2020 at 09:00:18AM +0200, Jürgen Groß wrote:
>> 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.
> 
> Thanks, I assume no further action is needed on my side.

Right.


Juergen