bad caching behavior in 2.6.12rc1

[Bill Nottingham]

When I upgraded from 2.6.11 to 2.6.12rc1, the VM started
behaving really badly with respect to caching.

Test box is a 1.5GB laptop.

In typical use, I would open a mailbox A, and then switch
to mailbox B. Immediately switching back to mailbox A, I
would find out it was no longer cached. (Using maildirs,
FWIW.)

Looking at /proc/meminfo, I would see:

LowFree: 8-12MB
HighFree: 300-400MB
Cached: 100-200MB

i.e., it was evicting cache when there was plenty of highmem
available for use.

When going back to 2.6.11, and using the same sort of the
workload, the caching would behave more as expected.

Bill
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