Re: What is the point of the wbc->for_kupdate check in btree_writepages

[Chris Mason <clm@meta.com>]

On 5/8/25 2:06 AM, Christoph Hellwig wrote:
> Hi Chris,
> 
> you added a wbc->for_kupdate check to btree_writepages that skips the
> write in this case in commit 448d640b668d ("Btrfs: Fine tune the btree
> writeback exclusion some more") which is not associated without any
> information but the subject line.
> 
> Do you by any chance remember why it was doing that just for kupdate
> and not any other background writeback?  I'm asking because it is one
> of only two checks for this in file system code, and the other one
> in nfs at least checks for all background writeback and thus makes
> some sense to me.

Hi Christoph,

btrfs can keep changing dirty tree blocks in memory, but once we write
them, we have to recow.  Between transaction writeback kicking in every
30 seconds and us calling balance_dirty_pages on the btree inode,
kupdate was doing more harm than good (back in 2007).

Is the goal to get rid of for_kupdate?  I wonder if we can just flag the
btree inode to exclude from kupdate, or keep it off whatever list
kupdate cares about etc.

-chris