Re: [Xen-devel] Vmap allocator fails to allocate beyond 128MB

[Vijay Kilari <vijay.kilari@xxxxxxxxx>]

On Mon, Sep 29, 2014 at 12:11 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>> On 29.09.14 at 07:42, <vijay.kilari@xxxxxxxxx> wrote:
>> On Fri, Sep 26, 2014 at 9:21 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>>> On 26.09.14 at 17:23, <vijay.kilari@xxxxxxxxx> wrote:
>>>> In the call map_pages_to_xen() after for loop is performing the mapping
>>>> for next vm_bitmap pages. In case of arm  this call will set valid bit
>>>> is set to 1 in pte
>>>> entry for this mapping.
>>>
>>> So _that_ is the bug then, because ...
>>>
>>>> void __init vm_init(void)
>>>> {
>>>>      ....
>>>>      for ( i = 0, va = (unsigned long)vm_bitmap; i < nr; ++i, va += 
>>>> PAGE_SIZE
>> )
>>>>      {
>>>>          struct page_info *pg = alloc_domheap_page(NULL, 0);
>>>>
>>>>          map_pages_to_xen(va, page_to_mfn(pg), 1, PAGE_HYPERVISOR);
>>>>          clear_page((void *)va);
>>>>      }
>>>>      bitmap_fill(vm_bitmap, vm_low);
>>>>
>>>>      /* Populate page tables for the bitmap if necessary. */
>>>>      map_pages_to_xen(va, 0, vm_low - nr, MAP_SMALL_PAGES);
>>>
>>> ... here we don't request any valid leaf entries to be created. All we
>>> want are the non-leaf page table structures.
>>
>> Do we need to reserve this non-leaf page table structures?
>> If vm_low is reserving the virtual address, vm_alloc will not allocate
>> vm_bitmap reserved pages.
>
> Not sure what you mean with "reserve" here. Again - we want the
> page table structures to be created (without handing out the
> respective virtual addresses to anyone), so that we don't need to
> be bothered with this when actual allocation requests come in.

I mean, vm_low holds the lowest clear bit in vm_bitmap from where
vm_alloc starts allocation. By setting vm_low = PFN_UP((vm_end + 7) / 8);
vm_low is reserving the all the virtual address/pages required for
vm_bitmap in future.

Ex: In case of 1GB vmap space. vm_low is 8. meaning the first
8 pages are reserved for vm_bitmap.

We don't need to allocate page table structures in vm_init
for vm_bitmap reserved pages  because this  virtual addresses are
reserved for vm_bitmap and are not given to anyone by vm_alloc.
So page table structures can be created when actual allocation
requests comes in for vm_bitmap.

So I feel map_pages_to_xen() after for loop in vm_init
is not required.

Vijay

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