Re: [PATCH 05/12] xen: introduce reserve_heap_pages

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.04.2020 18:21, Stefano Stabellini wrote:
> On Thu, 30 Apr 2020, Jan Beulich wrote:
>> On 30.04.2020 00:46, Stefano Stabellini wrote:
>>> On Fri, 17 Apr 2020, Jan Beulich wrote:
>>>> On 15.04.2020 03:02, Stefano Stabellini wrote:
>>>>> Introduce a function named reserve_heap_pages (similar to
>>>>> alloc_heap_pages) that allocates a requested memory range. Call
>>>>> __alloc_heap_pages for the implementation.
>>>>>
>>>>> Change __alloc_heap_pages so that the original page doesn't get
>>>>> modified, giving back unneeded memory top to bottom rather than bottom
>>>>> to top.
>>>>
>>>> While it may be less of a problem within a zone, doing so is
>>>> against our general "return high pages first" policy.
>>>
>>> Is this something you'd be OK with anyway?
>>
>> As a last resort, maybe. But it really depends on why it needs to be
>> this way.
>>
>>> If not, do you have a suggestion on how to do it better? I couldn't find
>>> a nice way to do it without code duplication, or a big nasty 'if' in the
>>> middle of the function.
>>
>> I'd first need to understand the problem to solve.
> 
> OK, I'll make an example.
> 
> reserve_heap_pages wants to reserve the range 0x10000000 - 0x20000000.
> 
> reserve_heap_pages get the struct page_info for 0x10000000 and calls
> alloc_pages_from_buddy to allocate an order of 28.
> 
> alloc_pages_from_buddy realizes that the free memory area starting from
> 0x10000000 is actually of order 30, even larger than the requested order
> of 28. The free area is 0x10000000 - 0x50000000.
> 
> alloc_pages_from_buddy instead of just allocating an order of 28
> starting from 0x10000000, it would allocate the "top" order of 28 in the
> free area. So it would allocate: 0x40000000 - 0x50000000, returning
> 0x40000000.
> 
> Of course, this doesn't work for reserve_heap_pages.
> 
> 
> This patch changes the behavior of alloc_pages_from_buddy so that in a
> situation like the above, it would return 0x10000000 - 0x20000000
> (leaving the rest of the free area unallocated.)

So what if then, for the same order-30 (really order-18 if I assume
you name addresses, not frame numbers), a reservation request came
in for the highest order-28 sub-region? You'd again be screwed if
you relied on which part of a larger buddy gets returned by the
lower level function you call. I can't help thinking that basing
reservation on allocation functions can't really be made work for
all possible cases. Instead reservation requests need to check that
the requested range is free _and_ split the potentially larger
range according to the request.

Jan