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

[Christoph Hellwig <hch@infradead.org>]

On Thu, May 08, 2025 at 10:33:24AM -0400, Chris Mason wrote:
> 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).

I totally understand why you'd want to avoid background writeback.  I
just don't understand why it singles out for_kupdate.

> Is the goal to get rid of for_kupdate?

At least getting rid of exposing it to file systems, yes.

> I wonder if we can just flag the
> btree inode to exclude from kupdate, or keep it off whatever list
> kupdate cares about etc.

Not having the VM do writeback on metadata but running it from a fs
LRU was a huge win in XFS.  I'm not sure we have interfaces that keep
data in the pagecache but never do any background writeback.  But
if you are fine with treating all background writeback equal that
would be exactly where I'd like to go to.