Re: [RFC][PATCH] Fix area->nr_free-- went (-1) issue in buddy system

From: Nick Piggin
Date: Mon Dec 18 2006 - 23:48:04 EST


Aubrey wrote:
Hi all,

When I setup two zones (NORMAL and DMA) in my system, I got the
following wired result from /proc/buddyinfo.
-----------------------------------------------------------------------------------------

root:~> cat /proc/buddyinfo
Node 0, zone DMA 2 1 2 1 1 0 0
1 1 2 2 0 0 0
Node 0, zone Normal 1 1 1 1 1 1 0
0 4294967295 0 4294967295 2 0 0
-----------------------------------------------------------------------------------------


As you see, two area->nr_free went -1.

After dig into the code, I found the problem is in the fun
__free_one_page() when the kernel boot up call free_all_bootmem(). If
two zones setup, it's possible NORMAL zone merged a block whose order
=8 at the first time(this time zone[NORMA]->free_area[8].nr_free = 0)
and found its buddy in the DMA zone. So the two blocks will be merged
and area->nr_free went to -1.

This should not happen because the pages are checked to ensure they are
from the same zone before merging.

What kind of system do you have? What is the dmesg and the .config? It
could be that the zones are not properly aligned and CONFIG_HOLES_IN_ZONE
is not set.

--
SUSE Labs, Novell Inc.
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