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SubjectRe: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?
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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