linux-kernel - Re: [Xen-devel] [PATCH 06/13] xen: detect pre-allocated memory interfering with e820 map

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Date:	Wed, 25 Feb 2015 17:00:07 +0100
From:	Juergen Gross <jgross@...e.com>
To:	David Vrabel <david.vrabel@...rix.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	konrad.wilk@...cle.com, boris.ostrovsky@...cle.com
Subject: Re: [Xen-devel] [PATCH 06/13] xen: detect pre-allocated memory interfering
 with e820 map

On 02/25/2015 03:24 PM, David Vrabel wrote:
> On 24/02/15 06:27, Juergen Gross wrote:
>> On 02/19/2015 07:07 PM, David Vrabel wrote:
>>> On 18/02/2015 06:51, Juergen Gross wrote:
>>>> +{
>>>> +    unsigned long pfn;
>>>> +    unsigned long area_start, area_end;
>>>> +    unsigned i;
>>>> +
>>>> +    for (i = 0; i < XEN_N_RESERVED_AREAS; i++) {
>>>> +
>>>> +        if (!xen_reserved_area[i].size)
>>>> +            break;
>>>> +
>>>> +        area_start = PFN_DOWN(xen_reserved_area[i].start);
>>>> +        area_end = PFN_UP(xen_reserved_area[i].start +
>>>> +                  xen_reserved_area[i].size);
>>>> +        if (area_start >= end_pfn || area_end <= start_pfn)
>>>> +            continue;
>>>> +
>>>> +        if (area_start > start_pfn)
>>>> +            xen_set_identity_and_remap_chunk(start_pfn, area_start,
>>>> +                             released, remapped);
>>>> +
>>>> +        if (area_end < end_pfn)
>>>> +            xen_set_identity_and_remap_chunk(area_end, end_pfn,
>>>> +                             released, remapped);
>>>> +
>>>> +        *remapped += min(area_end, end_pfn) -
>>>> +                max(area_start, start_pfn);
>>>> +
>>>> +        return;
>>>
>>> Why not defer the whole chunk that conflicts?  Or for that matter defer
>>> all this remapping to the last minute.
>>
>> There are two problems arising from this:
>>
>> - In the initrd case remapping would be deferred too long: the initrd
>>    data is still in use when device initialization is running. And we
>>    really want the remap to have happened before PCI space is being used.
>
> I'm not sure I understand what you're saying here.

I thought you wanted to defer the remapping to the point where the
initrd memory is no longer being used. But the suggestion below is
more clear.

>
> I'm suggesting:
>
> 1. Reserve all holes.
>
> 2. Relocate (if necessary) all modules (initrd, etc.) to regions that
> are RAM in the e820.
>
> 3. Rebuild the p2m in RAM.
>
> 4. Relocate frames from E820 holes/reserved to the end, free p2m pages
> from the holes and replacing them with the read-only 1:1 page (where
> possible).
>
>> - Delaying all remapping to the point where the new p2m list is in place
>>    would either result in a p2m list with all memory holes covered with
>>    individual entries as the new list is built with those holes still
>>    populated, ...
>>    The first option could easily waste significant amounts of memory (on
>>    my test machine with 1TB RAM this would have been about 1GB), while
>>    the second option would be performance critical.
>
> I don't understand how this wastes memory.   When you relocate the
> frames from the holes you can reclaim the p2m pages for the holes (and
> replace them with the r/o mapped identity p2m page).

Okay, this would work, I guess.

I'll have a try with some new patches...


Juergen
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