Bug#628718: linux-2.6: Swap on encrypted volume slows system to a crawl (not explicable by encryption overhead)

[SirJective <sirjective@gmail.com>]

Package: linux-2.6
Version: 2.6.38-2
Severity: important


I'm using swap space on an LVM volume on a LUKS-encrypted partition.
Swapping is ridiculously slow (< 1MB/s) whenever I swap to either the encrypted
swap partition, or some file inside an encrypted partition.
It is reasonably fast when swapping to a file on an unencrypted partition.

Normal writes (file copying, dd if=/dev/zero, etc) to the encrypted partition are
reasonably fast (~30MB/s), so the CPU overhead caused by encryption
cannot account for this.

[I posted a similar bug for 2.6.32 to the linux-image-*-686 package in squeeze, which
was probably a bad idea, so I'm posting this to the linux-2.6 source package,
where all the other bugs are posted.]

The real-world impact is that, for example, when I open a huge image in iceweasel
with other stuff already open in the background, the system virtually freezes
up for 15 minutes or more.



-- System Information:
Debian Release: wheezy/sid
  APT prefers testing
  APT policy: (500, 'testing')
Architecture: i386 (i686)

Kernel: Linux 2.6.38-2-686 (SMP w/1 CPU core)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash