Date: Mon, 17 Dec 2007 20:42:04 -0600
From: Matt Mackall <mpm@selenic.com>
To: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Mel Gorman <mel@csn.ul.ie>, Andrew Morton <akpm@linux-foundation.org>,
       James Bottomley <James.Bottomley@HansenPartnership.com>,
       jens.axboe@oracle.com, liml@rtr.ca, lkml@rtr.ca, matthew@wil.cx,
       linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org,
       linux-mm@kvack.org
Subject: Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?
Message-ID: <20071218024204.GR19691@waste.org>
References: <20071213200219.GI10104@kernel.dk> <476190BE.9010405@rtr.ca> <20071213200958.GK10104@kernel.dk> <20071213140207.111f94e2.akpm@linux-foundation.org> <1197584106.3154.55.camel@localhost.localdomain> <20071213142935.47ff19d9.akpm@linux-foundation.org> <20071215010940.GB28613@csn.ul.ie> <20071214180206.e0325503.akpm@linux-foundation.org> <20071216215519.GA7710@csn.ul.ie> <20071217112457.d7940c63.randy.dunlap@oracle.com>
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On Mon, Dec 17, 2007 at 11:24:57AM -0800, Randy Dunlap wrote:
> On Sun, 16 Dec 2007 21:55:20 +0000 Mel Gorman wrote:
> 
> > > > Just using cp to read the file is enough to cause problems but I included
> > > > a very basic program below that produces the BUG_ON checks. Is this a known
> > > > issue or am I using the interface incorrectly?
> > > 
> > > I'd say you're using it correctly but you've found a hitherto unknown bug. 
> > > On i386 highmem machines with CONFIG_HIGHPTE (at least) pte_offset_map()
> > > takes kmap_atomic(), so pagemap_pte_range() can't do copy_to_user() as it
> > > presently does.
> > > 
> > > Drat.
> > > 
> > > Still, that shouldn't really disrupt the testing which you're doing.  You
> > > could disable CONFIG_HIGHPTE to shut it up.
> > > 
> > 
> > Yes, that did the trick. Using pagemap, it was trivial to show that the
> > 2.6.24-rc5-mm1 kernel was placing pages in reverse physical order like
> > the following output shows
> > 
> > b:  32763 v:   753091 p:    65559 . 65558 contig: 1
> > b:  32764 v:   753092 p:    65558 . 65557 contig: 1
> > b:  32765 v:   753093 p:    65557 . 65556 contig: 1
> > b:  32766 v:   753094 p:    65556 . 65555 contig: 1
> > b:  32767 v:   753095 p:    65555 . 65555 contig: 1
> > 
> > p: is the PFN of the page v: is the page offset within an anonymous
> > mapping and b: is the number of non-contiguous blocks in the anonymous
> > mapping. With the patch applied, it looks more like;
> > 
> > b:   1232 v:   752964 p:    58944 ................ 87328 contig: 15
> > b:   1233 v:   752980 p:    87328 ................ 91200 contig: 15
> > b:   1234 v:   752996 p:    91200 ................ 40272 contig: 15
> > b:   1235 v:   753012 p:    40272 ................ 85664 contig: 15
> > b:   1236 v:   753028 p:    85664 ................ 87312 contig: 15
> > 
> > so mappings are using contiguous pages again. This was the final test
> > program I used in case it's of any interest.
> > 
> > Thanks
> > 
> > /*
> >  * showcontiguous.c
> >  *
> >  * Use the /proc/pid/pagemap interface to give an indication of how contiguous
> >  * physical memory is in an anonymous virtual memory mapping
> >  */
> 
> Matt,
> Did you ever make your python pagemap scripts available?
> If not, would you?

There's a collection of them at http://selenic.com/repo/pagemap.
They're largely proof of concept, and I'm not sure I finished adapting
them all to the final 64-bit interface.

As it happens, the above regression I actually spotted immediately by
doing a simple hexdump on my very first test of the interface - lots
of pfns counting backwards. Mentioned it a few times to various people
in the cc: list and on lkml but never got around to tracking it down
myself..

-- 
Mathematics is the supreme nostalgia of our time.
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