something is dramatically wrong with buffered read performance in 2.6.32.9

I recently upgraded from kernel 2.6.31.1 to 2.6.32.9. Upon kicking the tires on the new kernel I discovered that my two data partitions are suffering a ~30MB/s performance loss compared to my root partition. That's a loss of nearly half the performance of my disk.

The root partition is a primary, formatted with EXT2. The two data partitions are logicals within an extended partition, both formatted with XFS. hdparm O_DIRECT tests show identical throughput for all 3 partitions.
Going through the buffer cache, however, shows a huge performance drop for
the two logical partitions. Throughput is nearly cut in half through the buffer cache:

/dev/sda2:
Timing O_DIRECT disk reads: 238 MB in 3.02 seconds = 78.79 MB/sec
/dev/sda2:
Timing buffered disk reads: 208 MB in 3.00 seconds = 69.26 MB/sec

/dev/sda6:
Timing O_DIRECT disk reads: 236 MB in 3.02 seconds = 78.14 MB/sec
/dev/sda6:
Timing buffered disk reads: 126 MB in 3.01 seconds = 41.79 MB/sec

/dev/sda7:
Timing O_DIRECT disk reads: 238 MB in 3.00 seconds = 79.27 MB/sec
/dev/sda7:
Timing buffered disk reads: 126 MB in 3.03 seconds = 41.65 MB/sec

Any ideas what I'm running into here? Did I somehow fubar something in menuconfig?

I get the same results with dd copy tests through the buffer cache to /dev/null. Any idea what's wrong? Is this a known bug?

Stan