2.6.24-rc5-mm1 - wonky disk cache and CDROM behavior...

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On Thu, 13 Dec 2007 02:40:50 PST, Andrew Morton said:
> ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.24-rc5/2.6.24-rc5-mm1/

OK, so I'm trying to 'dd' a CD and the drive on the laptop is having issues
reading the disk.

I try it once, and get an I/O error about 117M in - dd reports 1.7M/sec.

I try it again, and it reports it died at the same exact place, but in about
2 seconds flat, and reports 91M/sec transfer.  OK, that's *weird*, I didn't
think that blocks read from /dev/cdrom would get cached, but OK.  So I try
the obviously stupid thing:

# echo 1 >| /proc/sys/vm/drop_caches

Alas, that hangs gloriously - 'echo t > /proc/sysrq-trigger' tells me:

Dec 17 17:30:02 turing-police kernel: [20235.823201] bash          D 0000000000000001  5288 15123  15085
Dec 17 17:30:02 turing-police kernel: [20235.823206]  ffff81007ba7de28 0000000000000086 0000000000000000 0000000000000000
Dec 17 17:30:02 turing-police kernel: [20235.823210]  ffff81007bbd9000 ffff81007d70e000 ffff81007bbd9248 00000001019e3e48
Dec 17 17:30:02 turing-police kernel: [20235.823214]  ffffe20000f36028 ffffe200012b9978 ffffe20000eece48 ffffe20001164188
Dec 17 17:30:02 turing-police kernel: [20235.823218] Call Trace:
Dec 17 17:30:02 turing-police kernel: [20235.823224]  [<ffffffff80523e20>] __down_read+0x87/0xa1
Dec 17 17:30:02 turing-police kernel: [20235.823229]  [<ffffffff8024bc13>] down_read+0x9/0xe
Dec 17 17:30:02 turing-police kernel: [20235.823232]  [<ffffffff802abafe>] drop_pagecache+0x3a/0x8c
Dec 17 17:30:02 turing-police kernel: [20235.823235]  [<ffffffff802abb72>] drop_caches_sysctl_handler+0x22/0x38
Dec 17 17:30:02 turing-police kernel: [20235.823239]  [<ffffffff802d2b70>] proc_sys_write+0x7e/0xa6
Dec 17 17:30:02 turing-police kernel: [20235.823244]  [<ffffffff8028e18c>] vfs_write+0xc7/0x170
Dec 17 17:30:02 turing-police kernel: [20235.823248]  [<ffffffff8028e772>] sys_write+0x47/0x70
Dec 17 17:30:02 turing-police kernel: [20235.823251]  [<ffffffff8020c34c>] tracesys+0xdc/0xe1

Yowza.  And no, I don't dare to try to reproduce it until Thursday at the
earliest, I need a mostly-stable machine the next 2 days. ;)

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