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Re: [Xen-devel] domU oom -> xvda1 read-only without any notice?

To: Josip Rodin <joy@xxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] domU oom -> xvda1 read-only without any notice?
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 31 Mar 2010 09:45:01 +0100
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On 31/03/2010 08:49, "Josip Rodin" <joy@xxxxxxxxxxxxxx> wrote:

>> Blkback communicates this kind of info to blkfront via a node in xenstore
>> named 'info'. Bit 2 of this numeric field indicates if a virtual disc is
>> read-only. I think it's more likely that the domU has internally confused
>> itself, rather than being told to mount read-only by dom0 -- the flags get
>> probed when the virtual disc first appears, and I don't think would get
>> probed again after that anyway without a full disc hot-unplug/replug.
> OK, thanks, so is there any way I could examine xen-blkfront then? :)
> I looked around and only found funny bits like:
> % cat /sys/module/xen_blkfront/drivers/xen:vbd/vbd-51713/block/xvda1/ro
> 0

AFAIK that means that the read-only-ness is not being propagated up to domU
block layer by the xen_blockfront driver. I'm not sure exactly what other
possibilities there are, but if the kernel has been OOMing processes then
perhaps you're in a runlevel or mode, or even a kenrel bug, in which rootfs
is forced read-only for other reasons? There have been bugs around OOM in
the past, and really it's a kernel path that's obviously best avoided! Any
idea why the OOM occurred in the first place?

 -- Keir

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