Re: current git kernel has strange problems during bisect

[Linus Torvalds]

On Sun, 11 Jan 2009, Christian Borntraeger wrote:
> 
> I think we should really avoid merging subtrees to the linux kernel. It 
> makes bisecting a real PITA. Furthermore, It is unlikely, but what if 
> the problem is part of the 581 changesets from btrfs?

Umm, yes? 

The thing is, btrfs was developed as an outside module. There are two 
choices: import it with history, or import it without history. The history 
is interesting, so importing _with_ it is a much nicer one. But that does 
mean that btrfs introduces into the kernel tree the same behaviour we've 
had in the git development tree for a long time - multiple root commits, 
and "independent" branches that get merged.

It's actually very natural for git, and the btrfs tree actually was 
re-done with "git filter-branch" to move all the history so that it is in 
fs/btrfs, rather than moving around from the root like the _original_ 
development was done. So it's not technically a subtree merge, it's a 
regular merge with just two different root commits - one for the original 
base kernel development, one for the original btrfs kernel development.

For bisect, it's indeed somewhat annoying, and we could have perhaps done 
some things a bit differently, but it's about the closest you can get to 
"real history" without making the first btrfs merge-point a _total_ 
disaster.

For bisect purposes, if you know you're not chasing down a btrfs issue, 
you can do

	git bisect good 34353029534a08e41cfb8be647d734b9ce9ebff8

where that commit 34353029 is the last one which has _just_ the btrfs 
files. The next commit is when it does "Merge Btrfs into fs/btrfs", and 
that one has the whole kernel tree again.

			Linus
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