Re: [PATCH 4/4] xfs: constrain dirty buffers while formatting a staged btree

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Nov 28, 2023 at 09:50:35PM -0800, Christoph Hellwig wrote:
> On Tue, Nov 28, 2023 at 12:11:33PM -0800, Darrick J. Wong wrote:
> > And now that I've dug further through my notes, I've realized that
> > there's a better reason for this unexplained _get_buf -> _read_buf
> > transition and the setting of XBF_DONE in _delwri_queue_here.
> > 
> > This patch introduces the behavior that we flush the delwri list to disk
> > every 256k.
> 
> Where "the delwri list" is the one used for writing stage btrees I
> think.

Correct.

> > Flushing the buffers releases them, which means that
> > reclaim could free the buffer before xfs_btree_bload_node needs it again
> > to build the next level up.
> 
> Oh, indeed.
> 
> > If that's the case, then _get_buf will get
> > us a !DONE buffer with zeroes instead of reading the (freshly written)
> > buffer back in from disk.  We'll then end up formatting garbage keys
> > into the node block, which is bad.
> 
> Yeah.
> 
> > 		/*
> > 		 * Read the lower-level block in case the buffer for it has
> > 		 * been reclaimed.  LRU refs will be set on the block, which is
> > 		 * desirable if the new btree commits.
> > 		 */
> > 		ret = xfs_btree_read_buf_block(cur, child_ptr, 0, &child_block,
> > 				&child_bp);
> > 
> > The behavior of setting XBF_DONE in xfs_buf_delwri_queue_here is an
> > optimization if _delwri_submit releases the buffer and it is /not/
> > reclaimed.  In that case, xfs_btree_read_buf_block will find the buffer
> > without the DONE flag set and reread the contents from disk, which is
> > unnecessary.
> 
> Yeah.  I still find it weird to set it in the delwri_submit_here helper,
> but maybe that's a discussion for the other thread.

D'Oh!  XBF_DONE exists in userspace too, because libxfs uses it.  Well
then, the proper place for it is at the top of xfs_btree_bload_drop_buf.
I'll go change that tomorrow.

--D