Re: [PATCH 1/4] btrfs: backref, only collect file extent items matching backref offset

[David Sterba <dsterba@suse.cz>]

On Wed, Feb 12, 2020 at 08:11:56PM +0800, Qu Wenruo wrote:
> >>>
> >>> This looks like an existing bug, IIRC Zygo reported it before.
> >>>
> >>> Btrfs balance just randomly failed at data reloc tree.
> >>>
> >>> Thus I don't believe it's related to Ethan's patches.
> >>
> >> Ok, than the patches make it more likely to happen, which could mean
> >> that faster backref processing hits some race window. As there could be
> >> more we should first fix the bug you say Zygo reported.
> > 
> > I added a log to check if find_parent_nodes is ever called under
> > test btrfs/125. It turns out that btrfs/125 doesn't pass through the
> > function. What my patches do is all under find_parent_nodes.
> 
> Balance goes through its own backref cache, thus it doesn't utilize the
> path you're modifying.
> 
> So don't worry your patches look pretty good.
> 
> Furthermore, this csum mismatch is not related to backref walk, but the
> data csum and the data in data reloc tree, which are all created by balance.
> 
> So there is really no reason to block such good optimization.

I don't mean to block the patchset but when I test patchsets from 5
people and tests start to fail I need to know what's the cause and if
there's a fix in sight. So far the test failed 2 out of 2 (once the
branch itself and then with for-next), I can do more rounds but at this
point it's too reliable to reproduce so there is some connection.

Sometimes it looks like I blame the messenger and complaining under
patches that don't cause the bugs, but often I don't have anyting better
than new warnings between 2 test rounds. Once we have more eyes on the
problem we'll narrow it down and find the root cause.