linux-kernel - Re: [PATCH] xen/balloon: Support xend-based toolstack take two

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Date:   Fri, 17 Jan 2020 12:36:50 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     Jürgen Groß <jgross@...e.com>
Cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Stefano Stabellini <sstabellini@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] xen/balloon: Support xend-based toolstack take two

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.

Jan