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

[Matthew Rushton <mvrushton@xxxxxxxxx>]

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.

> > > This change more gracefully handles the dom0 use case and returns
> > > contiguous memory for subsequent allocations.
> > >
> > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> > > Cc: Keir Fraser <keir@xxxxxxx>
> > > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > > Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > > Cc: Tim Deegan <tim@xxxxxxx>
> > > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > > Signed-off-by: Matt Rushton <mrushton@xxxxxxxxxx>
> > > Signed-off-by: Matt Wilson <msw@xxxxxxxxxx>
> > > ---
> > >   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;
> > > +        pg2 = pg + (1 << --j);
> > > +        PFN_ORDER(pg) = j;
> > >           page_list_add_tail(pg, &heap(node, zone, j));
> > > -        pg += 1 << j;
> > AFAICT this uses the low half (pg) for the allocation _and_ puts it on
> > the freelist, and just leaks the high half (pg2).  Am I missing something?
> Argh, oops. this is totally my fault (not Matt R.'s). I ported the
> patch out of our development tree incorrectly. The code should have
> read:
>
>       while ( j != order )
>       {
>           struct page_info *pg2;
>
>           pg2 = pg + (1 << --j);
>           PFN_ORDER(pg2) = j;
>           page_list_add_tail(pg2, &heap(node, zone, j));
>       }
>
> Apologies to Matt for my mangling of his patch (which also already had
> the correct blank line per Andy's comment).
>
> --msw
No worries I was about to correct you:)


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