Re: Best way to segments/requests

[Mark Lord <liml@rtr.ca>]

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Adrian McMenamin wrote:
> On 14/12/2007, Tejun Heo <htejun@gmail.com> wrote:
>> Hello,
> 
>> There just isn't much room for maneuver w/ just one segment.  Large
>> contiguous memory region isn't too common these days.  That said, there
>> was a bug recently spotted by Mark Lord which made contiguous memory
>> regions even rarer.  Which kernel version are you using?
>>
> 
> Bang up to date latest git, ie -rc5-gitX
..

Not in -git yet, but it is in -mm.
Attached here for your convenience.

>>> Is that right? What is the best way to go here?
>> If you can spare some memory and cpu cycles, preparing a contiguous
>> buffer and staging data there might help.  It will eat up some cpu
>> cycles but it won't be too much compared to PIO cycles.
>>
> 
> OK, I'll try it
..

That's probably your best bet, even though it will mean copying
to/from your big bounce buffer with each I/O.

The code could be clever, I suppose, and only bounce when the supplied
I/O region is smaller than XXX pages.

Cheers


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"Improved version", more similar to the 2.6.23 code:

Fix page allocator to give better chance of larger contiguous segments (again).

Signed-off-by: Mark Lord <mlord@pobox.com
---

--- old/mm/page_alloc.c	2007-12-13 19:25:15.000000000 -0500
+++ linux-2.6/mm/page_alloc.c	2007-12-13 19:43:07.000000000 -0500
@@ -760,7 +760,7 @@
 		struct page *page = __rmqueue(zone, order, migratetype);
 		if (unlikely(page == NULL))
 			break;
-		list_add(&page->lru, list);
+		list_add_tail(&page->lru, list);
 		set_page_private(page, migratetype);
 	}
 	spin_unlock(&zone->lock);