Re: [PATCH 10/28] i386: map enough initial memory to create lowmem mappings

[Eric W. Biederman]

Jeremy Fitzhardinge <jeremy@xxxxxxxx> writes:

> Eric W. Biederman wrote:
>> Jeremy did your kernel have PAE enabled?
>>
>> It just occurred to me that we have at all of the memory below 1M (say about
>> 512K) mapped and available to setup new mappings.
>>
>> The only way I can see a page fault happening is if you were using a PAE
>> enabled kernel (so you were not updating the current page tables) and
>> you have more than 256M of low memory, and we don't get any much extra
>> from always mapping 4M at a time.
>
> Yes, that's the situation.  PAE enabled, ~768MB of memory and a large
> kernel which mostly fills the 8M mapping.

And it just occurred to me PSE disabled, otherwise you would not have
needed more than 4 pages.  I supposed you were testing the Xen case.

That said I know for certain now why we have never seen this in
production because all cpus that fit this configuration also support
PSE.

So the way I think we should properly fix this is:
- Enable PAE early so we are updating our current page table.
- Pass a limit to the bootmem allocator so we do not get pages
  pass the point where we have setup page tables.
- Handle memory allocation failures.

However only the first part is necessary to solve the problem,
in practice even if you are using 4K pages.

Eric
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