linux-kernel - Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

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Date:	Thu, 13 Dec 2007 23:17:39 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	liml@....ca, lkml@....ca, matthew@....cx,
	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org, linux-mm@...ck.org,
	Mel Gorman <mel@....ul.ie>
Subject: Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

On Thu, Dec 13 2007, Andrew Morton wrote:
> On Thu, 13 Dec 2007 21:09:59 +0100
> Jens Axboe <jens.axboe@...cle.com> wrote:
> 
> >
> > OK, it's a vm issue,
> 
> cc linux-mm and probable culprit.
> 
> >  I have tens of thousand "backward" pages after a
> > boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
> > reverse. So it looks like that bug got reintroduced.
> 
> Bill Irwin fixed this a couple of years back: changed the page allocator so
> that it mostly hands out pages in ascending physical-address order.
> 
> I guess we broke that, quite possibly in Mel's page allocator rework.
> 
> It would help if you could provide us with a simple recipe for
> demonstrating this problem, please.

Basically anything involving IO :-). A boot here showed a handful of
good merges, and probably in the order of 100,000 descending
allocations. A kernel make is a fine test as well.

Something like the below should work fine - if you see oodles of these
basicaly doing any type of IO, then you are screwed.

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index e30b1a4..8ce3fcc 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -1349,6 +1349,10 @@ new_segment:
 				sg = sg_next(sg);
 			}
 
+			if (bvprv) {
+				if (page_address(bvec->bv_page) + PAGE_SIZE == page_address(bvprv->bv_page) && printk_ratelimit())
+					printk("page alloc order backwards\n");
+			}
 			sg_set_page(sg, bvec->bv_page, nbytes, bvec->bv_offset);
 			nsegs++;
 		}

-- 
Jens Axboe

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