Re: [PATCH V3 2/8] xen: Delay remapping memory of pv-domain

[Juergen Gross]

On 11/13/2014 08:56 PM, Konrad Rzeszutek Wilk wrote:
> > > > +	mfn_save = virt_to_mfn(buf);
> > > > +
> > > > +	while (xen_remap_mfn != INVALID_P2M_ENTRY) {
> > >
> > > So the 'list' is constructed by going forward - that is from low-numbered
> > > PFNs to higher numbered ones. But the 'xen_remap_mfn' is going the
> > > other way - from the highest PFN to the lowest PFN.
> > >
> > > Won't that mean we will restore the chunks of memory in the wrong
> > > order? That is we will still restore them in chunks size, but the
> > > chunks will be in descending order instead of ascending?
> >
> > No, the information where to put each chunk is contained in the chunk
> > data. I can add a comment explaining this.
>
> Right, the MFNs in a "chunks" are going to be restored in the right order.
>
> I was thinking that the "chunks" (so a set of MFNs) will be restored in
> the opposite order that they are written to.
>
> And oddly enough the "chunks" are done in 512-3 = 509 MFNs at once?

More don't fit on a single page due to the other info needed. So: yes.

> > > > +		/* Map the remap information */
> > > > +		set_pte_mfn(buf, xen_remap_mfn, PAGE_KERNEL);
> > > > +
> > > > +		BUG_ON(xen_remap_mfn != xen_remap_buf.mfns[0]);
> > > > +
> > > > +		free = 0;
> > > > +		pfn = xen_remap_buf.target_pfn;
> > > > +		for (i = 0; i < xen_remap_buf.size; i++) {
> > > > +			mfn = xen_remap_buf.mfns[i];
> > > > +			if (!released && xen_update_mem_tables(pfn, mfn)) {
> > > > +				remapped++;
> > >
> > > If we fail 'xen_update_mem_tables' we will on the next chunk (so i+1) keep on
> > > freeing pages instead of trying to remap. Is that intentional? Could we
> > > try to remap?
> >
> > Hmm, I'm not sure this is worth the effort. What could lead to failure
> > here? I suspect we could even just BUG() on failure. What do you think?
>
> I was hoping that this question would lead to making this loop a bit
> simpler as you would have to spread some of the code in the loop
> into functions.
>
> And keep 'remmaped' and 'released' reset every loop.
>
> However, if it makes the code more complex - then please
> forget my question.

Using BUG() instead would make the code less complex. Do you really
think xen_update_mem_tables() would ever fail in a sane system?

- set_phys_to_machine() would fail only on a memory shortage. Just
  going on without adding more memory wouldn't lead to a healthy system,
  I think.
- The hypervisor calls would fail only in case of parameter errors.
  This should never happen, so dying seems to be the correct reaction.

David, what do you think?


Juergen
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