Re: regression in ext4_ind_remove_space

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

On 01/30/2015 12:56 PM, Chris J Arges wrote:
> 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.

Well, after a bit more extensive testing a revert of this patch still
has failures. This may just be a novel bug.

I'll run longer tests on 77ea2a4b as well.
--chris

 I'm happy to
> continue debugging, or run any tests as necessary.
> 
> Thanks,
> --chris j arges
>