Re: [PATCH 1/3] fuse: remove reliance on bdi congestion

["NeilBrown" <neilb@suse.de>]

On Mon, 31 Jan 2022, Matthew Wilcox wrote:
> On Mon, Jan 31, 2022 at 03:03:53PM +1100, NeilBrown wrote:
> > diff --git a/fs/fuse/dax.c b/fs/fuse/dax.c
> > index 182b24a14804..5f74e2585f50 100644
> > --- a/fs/fuse/dax.c
> > +++ b/fs/fuse/dax.c
> > @@ -781,6 +781,9 @@ static int fuse_dax_writepages(struct address_space *mapping,
> >  	struct inode *inode = mapping->host;
> >  	struct fuse_conn *fc = get_fuse_conn(inode);
> >  
> > +	if (wbc->sync_mode == WB_SYNC_NONE &&
> > +	    fc->num_background >= fc->congestion_threshold)
> > +		return 0;
> >  	return dax_writeback_mapping_range(mapping, fc->dax->dev, wbc);
> 
> This makes no sense.  Doing writeback for DAX means flushing the
> CPU cache (in a terribly inefficient way), but it's not going to
> be doing anything in the background; it's a sync operation.

Fair enough ...  I was just being consistent.  I didn't wonder if dax
might be a bit special, but figured the change couldn't hurt.


> 
> > +++ b/fs/fuse/file.c
> > @@ -958,6 +958,8 @@ static void fuse_readahead(struct readahead_control *rac)
> >  
> >  	if (fuse_is_bad(inode))
> >  		return;
> > +	if (fc->num_background >= fc->congestion_threshold)
> > +		return;
> 
> This seems like a bad idea to me.  If we don't even start reads on
> readahead pages, they'll get ->readpage called on them one at a time
> and the reading thread will block.  It's going to lead to some nasty
> performance problems, exactly when you don't want them.  Better to
> queue the reads internally and wait for congestion to ease before
> submitting the read.
> 

Isn't that exactly what happens now? page_cache_async_ra() sees that
inode_read_congested() returns true, so it doesn't start readahead.
???

NeilBrown