Regression: Disk corruption with dm-crypt and kernels >= 4.0

[Abelardo Ricart III <aricart@memnix.com>]

I made sure to run a completely vanilla kernel when testing why I was suddenly
seeing some nasty libata errors with all kernels >= v4.0. Here's a snippet:

-------------------->8--------------------
[  165.592136] ata5.00: exception Emask 0x60 SAct 0x7000 SErr 0x800 action 0x6
frozen
[  165.592140] ata5.00: irq_stat 0x20000000, host bus error
[  165.592143] ata5: SError: { HostInt }
[  165.592145] ata5.00: failed command: READ FPDMA QUEUED
[  165.592149] ata5.00: cmd 60/08:60:a0:0d:89/00:00:07:00:00/40 tag 12 ncq 4096
in
                        res 40/00:74:40:58:5d/00:00:00:00:00/40 Emask 0x60
(host bus error)
[  165.592151] ata5.00: status: { DRDY }
-------------------->8--------------------

After a few dozen of these errors, I'd suddenly find my system in read-only
mode with corrupted files throughout my encrypted filesystems (seemed like
either a read or a write would corrupt a file, though I could be mistaken). I
decided to do a git bisect with a random read-write-sync test to narrow down
the culprit, which turned out to be this commit (part of a series):

# first bad commit: [cf2f1abfbd0dba701f7f16ef619e4d2485de3366] dm crypt: don't
allocate pages for a partial request

Just to be sure, I created a patch to revert the entire nine patch series that
commit belonged to... and the bad behavior disappeared. I've now been running
kernel 4.0 for a few days without issue, and went so far as to stress test my
poor SSD for a few hours to be 100% positive.

Here's some more info on my setup.

-------------------->8--------------------
$ lsblk -f
NAME         FSTYPE      LABEL MOUNTPOINT
sda                  
├─sda1       vfat              /boot/EFI
├─sda2       ext4              /boot
└─sda3       LVM2_member
  ├─SSD-root crypto_LUKS
  │ └─root   f2fs              /
  └─SSD-home crypto_LUKS
    └─home   f2fs              /home

$ cat /proc/cmdline
BOOT_IMAGE=/vmlinuz-linux-memnix cryptdevice=/dev/SSD/root:root:allow-discards
root=/dev/mapper/root acpi_osi=Linux security=tomoyo
TOMOYO_trigger=/usr/lib/systemd/systemd intel_iommu=on
modprobe.blacklist=nouveau rw quiet

$ cat /etc/lvm/lvm.conf | grep "issue_discards"
issue_discards = 1
-------------------->8--------------------

If there's anything else I can do to help diagnose the underlying problem, I'm
more than willing.

Thanks,

Abelardo Ricart.