Anomaly in Buddy bitmaps?

Anomaly in Buddy bitmaps?

[Alok Mooley]

I wrote a module in kernel 2.6.0 for scanning a higher
order block from zone_mem_map for ZONE_NORMAL &
checking the buddy bitmaps for the same.
       In the case of order 4, while scanning on the
order 4 block boundaries, I found an order 4 block
with page state 0000000001111111,where 0s represent
free pages & 1s represent order 0 allocations. The bit
in the order 3 bitmap corresponding to this 4th order
block was found to be a 0,whereas this bit should have
been a 1 as one 3rd order buddy is completely free.
I got the same result (a 0, where a 1 should have been
found) in another case too.
Is this an anomaly in the buddy bitmaps? Can the buddy
bitmaps ever be inconsistent?


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Re: Anomaly in Buddy bitmaps?

[William Lee Irwin III]

On Tue, Jan 20, 2004 at 11:57:29AM -0800, Alok Mooley wrote:
> I wrote a module in kernel 2.6.0 for scanning a higher
> order block from zone_mem_map for ZONE_NORMAL &
> checking the buddy bitmaps for the same.
>        In the case of order 4, while scanning on the
> order 4 block boundaries, I found an order 4 block
> with page state 0000000001111111,where 0s represent
> free pages & 1s represent order 0 allocations. The bit
> in the order 3 bitmap corresponding to this 4th order
> block was found to be a 0,whereas this bit should have
> been a 1 as one 3rd order buddy is completely free.
> I got the same result (a 0, where a 1 should have been
> found) in another case too.
> Is this an anomaly in the buddy bitmaps? Can the buddy
> bitmaps ever be inconsistent?

This could be the result of one of the free buddies being on the
per-cpu freelists. Count those as "semi-free"; they count as allocated
as far as the buddy bitmap is concerned.


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