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SubjectRe: [PATCH 2/5] Light Fragmentation Avoidance V20: 002_usemap
On Tue, 22 Nov 2005, Andy Whitcroft wrote:

> Mel Gorman wrote:
> > That's iterating through, potentially, 1024 pages which I considered too
> > expensive. In terms of code complexity, the page-flags patch adds 237
> > which is not much of a saving in comparison to 275 that the usemap
> > approach uses.
> Surley you would just use a single bit in the first page of a MAX_ORDER
> block. We guarentee that the mem_map is contigious out to MAX_ORDER
> pages so you can simply calculate the offset. The page free path does
> the same thing to find the buddy pages when coallescing.
> > Again, I can revisit the page-flag approach if I thought that something
> > like this would get merged and people would not choke on another page flag
> > being consumed.
> All of that said, I am not even sure we have a bit left in the page
> flags on smaller architectures :/.

Based on the 2.6.15-rc1-mm2, there is a macro FLAGS_RESERVED defined in
include/linux/mmzone.h which says how many bits are served for teh
node+zone bits in the page flags with the remainder for normal page flags.
It's currently 9, leaving 21 bits for page flags of which 19 are used. I
don't think using another bit will cause breakage but I imagine it will
make eyebrows furrow.

Mel Gorman
Part-time Phd Student Java Applications Developer
University of Limerick IBM Dublin Software Lab
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