regression in ext4_ind_remove_space

[Chris J Arges <chris.j.arges@canonical.com>]

Hi,

Users of non-extent ext4 filesystems (ext4 ^extents, or ext3 w/
CONFIG_EXT4_USE_FOR_EXT23=y) can encounter data corruption when using
fallocate with FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE flags. This
seems to be a regression in ext4_ind_remove_space introduced in
4f579ae7, whereas commit 77ea2a4b passes the following test case.

To reproduce this issue do the following:
1) Setup ext4 ^extents, or ext3 filesystem with CONFIG_EXT4_USE_FOR_EXT23=y
2) Create and install a VM using a qcow2 image and store the file on the
filesystem
3) Snapshot the image with qemu-img
4) Boot the image and do some disk operations (fio,etc)
5) Shutdown image and delete snapshot
6) Repeat 3-5 until VM no longer boots due to image corruption,
generally this takes a few iterations depending on disk operations.

In addition, I've tested this with a single vCPU and single host CPU and
the problem persists. Running the same test on ext4 w/ extents exhibits
no failures.

Any ideas for bug hunting here? Commit 4f579ae7 completely re-writes
ext4_ind_remove_space. A revert of that commit from master fixes the
issue, but I'm unsure if that un-fixes other things. I'm happy to
continue debugging, or run any tests as necessary.

Thanks,
--chris j arges