Re: [Xen-devel] [PATCHv1] xen/balloon: disable memory hotplug in PV guests

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 16/03/15 10:31, Juergen Gross wrote:
> On 03/16/2015 11:03 AM, Daniel Kiper wrote:
>> On Mon, Mar 16, 2015 at 06:35:04AM +0100, Juergen Gross wrote:
>>> On 03/11/2015 04:40 PM, Boris Ostrovsky wrote:
>>>> On 03/11/2015 10:42 AM, David Vrabel wrote:
>>>>> On 10/03/15 13:35, Boris Ostrovsky wrote:
>>>>>> On 03/10/2015 07:40 AM, David Vrabel wrote:
>>>>>>> On 09/03/15 14:10, David Vrabel wrote:
>>>>>>>> Memory hotplug doesn't work with PV guests because:
>>>>>>>>
>>>>>>>>     a) The p2m cannot be expanded to cover the new sections.
>>>>>>> Broken by 054954eb051f35e74b75a566a96fe756015352c8 (xen: switch to
>>>>>>> linear virtual mapped sparse p2m list).
>>>>>>>
>>>>>>> This one would be non-trivial to fix.  We'd need a sparse set of
>>>>>>> vm_area's for the p2m or similar.
>>>>>>>
>>>>>>>>     b) add_memory() builds page tables for the new sections which
>>>>>>>> means
>>>>>>>>        the new pages must have valid p2m entries (or a BUG occurs).
>>>>>>> After some more testing this appears to be broken by:
>>>>>>>
>>>>>>> 25b884a83d487fd62c3de7ac1ab5549979188482 (x86/xen: set regions above
>>>>>>> the
>>>>>>> end of RAM as 1:1) included 3.16.
>>>>>>>
>>>>>>> This one can be trivially fixed by setting the new sections in
>>>>>>> the p2m
>>>>>>> to INVALID_P2M_ENTRY before calling add_memory().
>>>>>> Have you tried 3.17? As I said yesterday, it worked for me (with 4.4
>>>>>> Xen).
>>>>> No.  But there are three bugs that prevent it from working in 3.16+ so
>>>>> I'm really not sure how you had a working in a 3.17 PV guest.
>>>>
>>>> This is what I have:
>>>>
>>>> [build@build-mk2 linux-boris]$ ssh root@tst008 cat
>>>> /mnt/lab/bootstrap-x86_64/test_small.xm
>>>> extra="console=hvc0 debug earlyprintk=xen "
>>>> kernel="/mnt/lab/bootstrap-x86_64/vmlinuz"
>>>> ramdisk="/mnt/lab/bootstrap-x86_64/initramfs.cpio.gz"
>>>> memory=1024
>>>> maxmem = 4096
>>>> vcpus=1
>>>> maxvcpus=3
>>>> name="bootstrap-x86_64"
>>>> on_crash="preserve"
>>>> vif = [ 'mac=00:0F:4B:00:00:68, bridge=switch' ]
>>>> vnc=1
>>>> vnclisten="0.0.0.0"
>>>> disk=['phy:/dev/guests/bootstrap-x86_64,xvda,w']
>>>> [build@build-mk2 linux-boris]$ ssh root@tst008 xl create
>>>> /mnt/lab/bootstrap-x86_64/test_small.xm
>>>> Parsing config from /mnt/lab/bootstrap-x86_64/test_small.xm
>>>> [build@build-mk2 linux-boris]$ ssh root@tst008 xl list |grep
>>>> bootstrap-x86_64
>>>> bootstrap-x86_64                             2  1024     1
>>>> -b----       5.4
>>>> [build@build-mk2 linux-boris]$ ssh root@g-pvops uname -r
>>>> 3.17.0upstream
>>>> [build@build-mk2 linux-boris]$ ssh root@g-pvops dmesg|grep
>>>> paravirtualized
>>>> [    0.000000] Booting paravirtualized kernel on Xen
>>>> [build@build-mk2 linux-boris]$ ssh root@g-pvops grep MemTotal
>>>> /proc/meminfo
>>>> MemTotal:         968036 kB
>>>> [build@build-mk2 linux-boris]$ ssh root@tst008 xl mem-set
>>>> bootstrap-x86_64 2048
>>>> [build@build-mk2 linux-boris]$ ssh root@tst008 xl list |grep
>>>> bootstrap-x86_64
>>>> bootstrap-x86_64                             2  2048     1
>>>> -b----       5.7
>>>> [build@build-mk2 linux-boris]$ ssh root@g-pvops grep MemTotal
>>>> /proc/meminfo
>>>> MemTotal:        2016612 kB
>>>> [build@build-mk2 linux-boris]$
>>>>
>>>>
>>>>>
>>>>> Regardless, it definitely doesn't work now because of the linear p2m
>>>>> change.  What do you want to do about this?
>>>>
>>>> Since backing out p2m changes is not an option I guess your patch is
>>>> the
>>>> only short-term alternative.
>>>>
>>>> But this still looks like a regression so perhaps Juergen can take a
>>>> look to see how it can be fixed.
>>>
>>> Hmm, the p2m list is allocated for the maximum memory size of the domain
>>> which is obtained from the hypervisor. In case of Dom0 it is read via
>>> XENMEM_maximum_reservation, for a domU it is based on the E820 memory
>>> map read via XENMEM_memory_map.
>>>
>>> I just tested it with a 4.0-rc1 domU kernel with 512MB initial memory
>>> and 4GB of maxmem. The E820 map looked like this:
>>>
>>> [    0.000000] Xen: [mem 0x0000000000000000-0x000000000009ffff] usable
>>> [    0.000000] Xen: [mem 0x00000000000a0000-0x00000000000fffff] reserved
>>> [    0.000000] Xen: [mem 0x0000000000100000-0x00000000ffffffff] usable
>>>
>>> So the complete 4GB were included, like they should. The resulting p2m
>>> list is allocated in the needed size:
>>>
>>> [    0.000000] p2m virtual area at ffffc90000000000, size is 800000
>>>
>>> So what is your problem here? Can you post the E820 map and the p2m map
>>> info for your failing domain, please?
>>
>> If you use memory hotplug then maxmem is not a limit from guest kernel
>> point of view (host still must allow that operation but it is another
>> not related issue). The problem is that p2m must be dynamically
>> expendable
>> to support it. Earlier implementation supported that thing and memory
>> hotplug worked without any issue.
> 
> Okay, now I get it.
> 
> The problem with the earlier p2m implementation was that it was
> expendable to support only up to 512GB of RAM. So we need some way to
> tell the kernel how much virtual memory it should reserve for the p2m
> list if memory hotplug is enabled. We could:
> 
> a) use a configurable maximum (e.g. for 512GB RAM as today)

I would set the p2m virtual area to cover up to 512 GB (needs 1 GB of
virt space) for a 64-bit guest and up to 64 GB (needs 64 MB of virt
space) for a 32-bit guest.

This fixes the regression with minimal complexity and reasonable overheads.

David

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