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Re: [Xen-devel] file corruption!!!

To: James Harper <JamesH@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] file corruption!!!
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Mon, 19 Jul 2004 08:30:33 +0100
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> I'm not in a position to test this, but is it possible that the corruption 
> problem could manifest itself after an out of memory condition? When I first 
> noticed the corruption I rebooted as quickly as possible so it didn't 
> continue and so didn't check, but it's possible that it ran out of memory 
> first. I guess I could test this but don't really want to do anything to risk 
> corruption any further :)
> speaking of memory, I have 3 domains running currently, 0 + 2U, all declared 
> with 128mb memory, but xm list shows this:
> Dom  Name             Mem(MB)  CPU  State  Time(s)
> 0    Domain-0             119    0  r----   1293.0
> 6    gaia                 127    1  -b---     81.9
> 7    mail2                126    0  -b---   1597.9
> 'free' under mail2 and gaia shows 128124 as the total amount of memory.
> I appreciate that maybe something about dom0 means that it shows something 
> different, but why would the other two report different amounts of memory 
> when they both have the same amount??? Both are running identical kernels.
> James

The backend network and blkdev drivers in DOM0 allocate multi-MB
chunks of memory, then free all the pages in that chunk back to Xen.
The chunks are then used for ephemeral mappings of I/O buffers from
frontend drivers in other guest OSes.

The total used for this is about 7 or 8MB, so DOM0's estimate of its
memory allocation will be thrown off by about that amount. It doesn't
realise that those large allocated chunks have had all their memory
released. :-)

 -- Keir

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