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

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Message-id: <164360446180.18996.6767388833611575467@noble.neil.brown.name>
Date:   Mon, 31 Jan 2022 15:47:41 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Matthew Wilcox" <willy@...radead.org>
Cc:     "Andrew Morton" <akpm@...ux-foundation.org>,
        "Jeff Layton" <jlayton@...nel.org>,
        "Ilya Dryomov" <idryomov@...il.com>,
        "Miklos Szeredi" <miklos@...redi.hu>,
        "Trond Myklebust" <trond.myklebust@...merspace.com>,
        "Anna Schumaker" <anna.schumaker@...app.com>, linux-mm@...ck.org,
        linux-nfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        ceph-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] fuse: remove reliance on bdi congestion

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