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

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Message-ID: <89348679-2db3-b1ea-f1e3-725493ac5ab8@ipsumj.de>
Date:   Mon, 02 Oct 2017 21:37:00 +0000
From:   HW42 <hw42@...umj.de>
To:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Cc:     boris.ostrovsky@...cle.com
Subject: Re: [Xen-devel] [PATCH v3] xen/balloon: don't online new memory
 initially

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.



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