Message-ID: <cb94e63d0812022302m341fcdf5wffbca0ec12db426b@mail.gmail.com>
Date:	Wed, 3 Dec 2008 09:02:49 +0200
From:	"wassim dagash" <wassim.dagash@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: KSWAPD Algorithm

Hi All,
Description:
I countered a weird problem with kswapd:
it runs in some infinite loop trying to swap until order 10 of zone
highmem is OK, While zone higmem (as I understand) has nothing to do
with contiguous physical memory (cause there is no 1-1 mapping) which
means kswapd will continue to try to balance order 10 of zone highmem
forever (or until someone release a very large chunk of highmem).
Can anyone please explain me the algorithm of kswapd and why it tries
to balance order 10 of zone higmem ?

Details:
I build an instrumented kernel with debug messages in
"zone_watermark_ok" function, and from the code and debug messages I
see that "zone_watermark_ok" returns 0 when kswapd invokes it (through
balance_pgdat) in order to decide if zone highmem is balanced or not,
which lead in some configurations to infinite loop of kswapd ( if no
large chunks of highmem released) . I added a condition to
"balance_pgdat" so it doesn't try to balance order higher than 1 in
zone highmem and this conditon solved the problem, what are the risks
with such solution? isn't it a bug that kswapd is looking for
continuous memory in zone highmem ( as I understand there is no 1-1
mapping in zone highmem which is meaningless in kswapd)?

Regards,
Wassim Dagash.
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