Re: [RFC PATCH] page_alloc: use first half of higher order chunks when halving

[Matthew Rushton <mvrushton@gmail.com>]

On 03/25/14 06:20, Matt Wilson wrote:
> On Tue, Mar 25, 2014 at 11:44:19AM +0000, Andrew Cooper wrote:
>> On 25/03/14 11:22, Matt Wilson wrote:
>>> From: Matt Rushton <mrushton@amazon.com>
>>>
>>> This patch makes the Xen heap allocator use the first half of higher
>>> order chunks instead of the second half when breaking them down for
>>> smaller order allocations.
>>>
>>> Linux currently remaps the memory overlapping PCI space one page at a
>>> time. Before this change this resulted in the mfns being allocated in
>>> reverse order and led to discontiguous dom0 memory. This forced dom0
>>> to use bounce buffers for doing DMA and resulted in poor performance.
>>>
>>> This change more gracefully handles the dom0 use case and returns
>>> contiguous memory for subsequent allocations.
>>>
>>> Cc: xen-devel@lists.xenproject.org
>>> Cc: Keir Fraser <keir@xen.org>
>>> Cc: Jan Beulich <jbeulich@suse.com>
>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>> Cc: Tim Deegan <tim@xen.org>
>>> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
>>> Signed-off-by: Matt Rushton <mrushton@amazon.com>
>>> Signed-off-by: Matt Wilson <msw@amazon.com>
>> How does dom0 work out that it is safe to join multiple pfns into a dma
>> buffer without the swiotlb?
> I'm not familiar enough with how this works to say. Perhaps Matt R.
> can chime in during his day. My guess is that xen_swiotlb_alloc_coherent()
> avoids allocating a contiguous region if the pages allocated already
> happen to be physically contiguous.
>
> Konrad, can you enlighten us? The setup code in question that does the
> remapping one page at a time is in arch/x86/xen/setup.c.
>

The swiotlb code will check if the underlying mfns are contiguous and 
use a bounce buffer if and only if they are not. Everything goes through 
the swiotlb via the normal Linux dma apis it's just a matter of if it 
uses a bounce buffer or not.


>>> ---
>>>   xen/common/page_alloc.c |    5 +++--
>>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
>>> index 601319c..27e7f18 100644
>>> --- a/xen/common/page_alloc.c
>>> +++ b/xen/common/page_alloc.c
>>> @@ -677,9 +677,10 @@ static struct page_info *alloc_heap_pages(
>>>       /* We may have to halve the chunk a number of times. */
>>>       while ( j != order )
>>>       {
>>> -        PFN_ORDER(pg) = --j;
>>> +        struct page_info *pg2;
>> At the very least, Xen style mandates a blank line after this variable
>> declaration.
> Ack.
>   
>> ~Andrew
>>
>>> +        pg2 = pg + (1 << --j);
>>> +        PFN_ORDER(pg) = j;
>>>           page_list_add_tail(pg, &heap(node, zone, j));
>>> -        pg += 1 << j;
>>>       }
>>>   
>>>       ASSERT(avail[node][zone] >= request);