Re: Post ext3 conversion problems

[Sean Greenslade <sean@seangreenslade.com>]

On Tue, Sep 20, 2016 at 10:49:42AM +0800, Qu Wenruo wrote:
> OK, I see the problem now.
> 
> The new convert is designed to create minimal number of extents, so it
> result the following file extents layout:
> 
> Ext2_save/image
>         |
> /-------------------------------\
> |           Extent A             |
> |<---Old Ext3 Used space  ------>|<--- Free space--->|
>       \---/\---/\---/
>         |    |    |
>        F1    F2   F3
> 
> This causes a large extent A, refered by ext2_save/image, and files like
> F1/2/3 just refer to part of the large extent A.
> 
> After removing the whole ext2_save subvolume, Extent A is still there, since
> F1/2/3 are still referring it.
> 
> |<---Old Ext3 Used space  ------>|<--- Free space--->|
> |           Extent A             |
>       \---/\---/\---/
>         |    |    |
>        F1    F2   F3
> 
> 
> 
> In that case, not balance but defrag is responsible to "split" the large
> extent and free the unused space.
> 
> However btrfs defrag doesn't work for case like subvolume or reflink for a
> long time, which leaves the extent layout unchanged.
> And balance doesn't handle it, as balance just relocated the large extent A
> and modify all referencers' pointer.
> 
> Sigh, I just forgot the fact that defrag doesn't work for a long time when
> designing the new convert.
> 
> So, we still need a working kernel defrag to make the fs to  be a "native"
> btrfs.
> 
> 
> However, such layout is completely valid for btrfs, one can generate it
> quite easily which following script:
> ----
> xfs_io -f -c "pwrite 0 16M" $mnt/orig
> for i in $(seq 0 15); do
> 	xfs_io -f -c "reflink $mnt/orig ${i}M 0 1M" $mnt/file${i}
> done
> ----
> 
> So there is still something wrong in your backtrace so that we need to dig
> further.
> 
> Any idea of your load pattern to trigger the bug?

Glad to hear you've found the core of the issue.

At this point, I can trigger it immediately. As soon as I log in and run
dmenu, it will attempt to rebuild its cache file (small text file that's
just a list of all executables in the PATH). Once that write happens,
the bug triggers and the fs goes read only.

--Sean