linux-kernel - Re: [RFC PATCH 02/53] netfs: Track the fpos above which the server has no data

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Date:   Mon, 16 Oct 2023 17:31:12 +0100
From:   David Howells <dhowells@...hat.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     dhowells@...hat.com, Steve French <smfrench@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Marc Dionne <marc.dionne@...istor.com>,
        Paulo Alcantara <pc@...guebit.com>,
        Ronnie Sahlberg <lsahlber@...hat.com>,
        Shyam Prasad N <sprasad@...rosoft.com>,
        Tom Talpey <tom@...pey.com>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Ilya Dryomov <idryomov@...il.com>,
        Christian Brauner <christian@...uner.io>,
        linux-afs@...ts.infradead.org, linux-cifs@...r.kernel.org,
        linux-nfs@...r.kernel.org, ceph-devel@...r.kernel.org,
        v9fs@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-cachefs@...hat.com
Subject: Re: [RFC PATCH 02/53] netfs: Track the fpos above which the server has no data

Jeff Layton <jlayton@...nel.org> wrote:

> >  (7) If stored data is culled from the local cache, we must set zero_point
> >      above that if the data also got written to the server.
> 
> When you say culled here, it sounds like you're just throwing out the
> dirty cache without writing the data back. That shouldn't be allowed
> though, so I must be misunderstanding what you mean here. Can you
> explain?

I meant fscache specifically.  Too many caches - and some of them with the
same names!

> >  (8) If dirty data is written back to the server, but not the local cache,
> >      we must set zero_point above that.
> 
> How do you write back without writing to the local cache? I'm guessing
> this means you're doing a non-buffered write?

I meant fscache.  fscache can decline to honour a request to store data.

> > +		if (size != i_size) {
> > +			truncate_pagecache(&vnode->netfs.inode, size);
> > +			netfs_resize_file(&vnode->netfs, size);
> > +			fscache_resize_cookie(afs_vnode_cache(vnode), size);
> > +		}
> 
> Isn't this an existing bug? AFS is not setting remote_i_size in the
> setattr path currently? I think this probably ought to be done in a
> preliminary AFS patch.

It is being set.  afs_apply_status() sets it.  This is called by
afs_vnode_commit_status() which is called from afs_setattr_success().  The
value isn't updated until we get the return status from the server that
includes the new value.

> > +	loff_t			zero_point;	/* Size after which we assume there's no data
> > +						 * on the server */
> 
> While I understand the concept, I'm not yet sure I understand how this
> new value will be used. It might be better to merge this patch in with
> the patch that adds the first user of this data.

I'll consider it.  At least it might make sense to move them adjacent to each
other in the series.

David