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

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Date:   Fri, 17 Jan 2020 12:01:02 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     Juergen 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 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.

Jan