Re: [PATCH v2 2/2] xen/setup: Remap Xen Identity Mapped RAM

[Matthew Rushton]

On 08/01/14 08:56, Boris Ostrovsky wrote:
> On 08/01/2014 11:52 AM, Matt Wilson wrote:
> > On Fri, Aug 01, 2014 at 03:52:28PM +0100, David Vrabel wrote:
> > > On 31/07/14 18:43, David Vrabel wrote:
> > > > On 20/07/14 01:01, Matt Rushton wrote:
> > > > > Instead of ballooning up and down dom0 memory this remaps the 
> > > > > existing mfns
> > > > > that were replaced by the identity map. The reason for this is 
> > > > > that the
> > > > > existing implementation ballooned memory up and and down which 
> > > > > caused dom0
> > > > > to have discontiguous pages. In some cases this resulted in the 
> > > > > use of bounce
> > > > > buffers which reduced network I/O performance significantly. This 
> > > > > change will
> > > > > honor the existing order of the pages with the exception of some 
> > > > > boundary
> > > > > conditions.
> > > > >
> > > > > To do this we need to update both the Linux p2m table and the Xen 
> > > > > m2p table.
> > > > > Particular care must be taken when updating the p2m table since 
> > > > > it's important
> > > > > to limit table memory consumption and reuse the existing leaf 
> > > > > pages which get
> > > > > freed when an entire leaf page is set to the identity map. To 
> > > > > implement this,
> > > > > mapping updates are grouped into blocks with table entries getting 
> > > > > cached
> > > > > temporarily and then released.
> > > > >
> > > > > On my test system before:
> > > > > Total pages: 2105014
> > > > > Total contiguous: 1640635
> > > > >
> > > > > After:
> > > > > Total pages: 2105014
> > > > > Total contiguous: 2098904
> > > > Applied to devel/for-linus-3.17
> > > Unfortunately, this produces too many WARNINGs on some boxes or
> > > with certain configurations.
> > Hi David,
> >
> > Do you have more information about the systems or configurations that
> > showed a problem?
>
>
> This appears to be happening on 32-bit dom0.
>
> -boris

It's unclear to me why the m2p update would be failing in this case (on 
first attempt) which causes the subsequent errors. I'll try to repro on 
a 32-bit dom0.

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