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Subject: Re: [PATCH] xen/balloon: Support xend-based toolstack take two
To:     Jan Beulich <jbeulich@suse.com>
Cc:     xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
        Stefano Stabellini <sstabellini@kernel.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        stable@vger.kernel.org
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Date:   Fri, 17 Jan 2020 12:45:48 +0100
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On 17.01.20 12:36, Jan Beulich wrote:
> On 17.01.2020 12:31, Jürgen Groß wrote:
>> On 17.01.20 12:01, Jan Beulich wrote:
>>> On 16.01.2020 18:00, Juergen Gross wrote:
>>>> Commit 3aa6c19d2f38be ("xen/balloon: Support xend-based toolstack")
>>>> tried to fix a regression with running on rather ancient Xen versions.
>>>> Unfortunately the fix was based on the assumption that xend would
>>>> just use another Xenstore node, but in reality only some downstream
>>>> versions of xend are doing that. The upstream xend does not write
>>>> that Xenstore node at all, so the problem must be fixed in another
>>>> way.
>>>>
>>>> The easiest way to achieve that is to fall back to the behavior before
>>>> commit 5266b8e4445c ("xen: fix booting ballooned down hvm guest")
>>>> in case the static memory maximum can't be read.
>>>
>>> I could use some help here: Prior to said commit there was
>>>
>>> 	target_diff = new_target - balloon_stats.target_pages;
>>>
>>>
>>> Which is, afaict, ...
>>>
>>>> --- a/drivers/xen/xen-balloon.c
>>>> +++ b/drivers/xen/xen-balloon.c
>>>> @@ -94,7 +94,7 @@ static void watch_target(struct xenbus_watch *watch,
>>>>    				  "%llu", &static_max) == 1))
>>>>    			static_max >>= PAGE_SHIFT - 10;
>>>>    		else
>>>> -			static_max = new_target;
>>>> +			static_max = balloon_stats.current_pages;
>>>>    
>>>>    		target_diff = (xen_pv_domain() || xen_initial_domain()) ? 0
>>>>    				: static_max - balloon_stats.target_pages;
>>>
>>> ... what the code does before your change. Afaict there was
>>> never a use of balloon_stats.current_pages in this function.
>>
>> That is a little bit indirect, yes. In the end I want static_max to
>> be either the maximum reported by Xen, or if not available, the current
>> assumed memory size, which can be found in balloon_stats.current_pages.
>>
>> The main idea is to avoid a negative target_diff which would result in
>> not ballooning down.
> 
> All understood. Yet the change is then not a restore of prior behavior
> (just in a limited case), but a change to behavior that we never there
> before. I.e. it was indeed my assumption that the code was right, but
> the description was misleading.

The description is misleading as it fails to mention commit
96edd61dcf44362d3e, which introduced target_diff. I'll add that to
the commit message.


Juergen