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

[Tim Deegan <tim@xxxxxxx>]

Hi,

At 13:09 -0700 on 25 Mar (1395749353), Matthew Rushton wrote:
> On 03/25/14 06:27, Matt Wilson wrote:
> > On Tue, Mar 25, 2014 at 01:19:22PM +0100, Tim Deegan wrote:
> >> At 13:22 +0200 on 25 Mar (1395750124), Matt Wilson wrote:
> >>> From: Matt Rushton <mrushton@xxxxxxxxxx>
> >>>
> >>> 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 seems like something better fixed on the dom0 side, by asking
> >> explicitly for contiguous memory in cases where it makes a difference.
> >> On the Xen side, this change seems harmless, but we might like to keep
> >> the explicitly reversed allocation on debug builds, to flush out
> >> guests that rely on their memory being contiguous.
> > Yes, I think that retaining the reverse allocation on debug builds is
> > fine. I'd like Konrad's take on if it's better or possible to fix this
> > on the Linux side.
> 
> I considered fixing it in Linux but this was a more straight forward 
> change with no downside as far as I can tell. I see no reason in not 
> fixing it in both places but this at least behaves more reasonably for 
> one potential use case. I'm also interested in other opinions.

Well, I'm happy enough with changing Xen (though it's common code so
you'll need Keir's ack anyway rather than mine), since as you say it
happens to make one use case a bit better and is otherwise harmless.
But that comes with a stinking great warning:

 - This is not 'fixing' anything in Xen because Xen is doing exactly
   what dom0 asks for in the current code; and conversely

 - dom0 (and other guests) _must_not_ rely on it, whether for
   performance or correctness.  Xen might change its page allocator at
   some point in the future, for any reason, and if linux perf starts
   sucking when that happens, that's (still) a linux bug.

Cheers,

Tim.


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