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

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Message-ID: <54658ABF.5050708@suse.com>
Date:	Fri, 14 Nov 2014 05:53:19 +0100
From:	Juergen Gross <jgross@...e.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	david.vrabel@...rix.com, boris.ostrovsky@...cle.com,
	x86@...nel.org, tglx@...utronix.de, mingo@...hat.com, hpa@...or.com
Subject: Re: [PATCH V3 2/8] xen: Delay remapping memory of pv-domain

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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