linux-kernel - Re: [Xen-devel] [PATCH v3] xen/balloon: don't online new memory initially

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Message-ID: <a35f18d7-3047-ab19-179c-470ea8f3ef3e@oracle.com>
Date:   Tue, 3 Oct 2017 17:33:25 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     HW42 <hw42@...umj.de>, Juergen Gross <jgross@...e.com>,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Subject: Re: [Xen-devel] [PATCH v3] xen/balloon: don't online new memory
 initially

On 10/02/2017 05:37 PM, HW42 wrote:
> Juergen Gross:
>> When setting up the Xenstore watch for the memory target size the new
>> watch will fire at once. Don't try to reach the configured target size
>> by onlining new memory in this case, as the current memory size will
>> be smaller in almost all cases due to e.g. BIOS reserved pages.
>>
>> Onlining new memory will lead to more problems e.g. undesired conflicts
>> with NVMe devices meant to be operated as block devices.
>>
>> Instead remember the difference between target size and current size
>> when the watch fires for the first time and apply it to any further
>> size changes, too.
>>
>> In order to avoid races between balloon.c and xen-balloon.c init calls
>> do the xen-balloon.c initialization from balloon.c.
>>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
> This patch seems to introduce a regression. If I boot a HVM or PVH
> domain with memory != maxmem then the kernel inside the domain reports
> that it has maxmem available even though Xen reports only what is set as
> memory. Sooner or later Xen logs "out of PoD memory!" and kills the
> domain. If I revert the corresponding commit (96edd61d) then everything
> works as expected.
>
> Tested this with Xen 4.9.0 and Linux 4.13.4.
>


Yes, this indeed doesn't look like it's doing the right thing (although
I haven't seen the "out of memory" error).

I wonder whether target_diff should be computed against xenstore's
"static-max" and not "target" and the memory should be ballooned down
immediately and not on a subsequent watch firing.

Unfortunately Juergen is out for a couple of weeks and I'd like to hear
from him first since he is the one who wrote this patch.

-boris



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