linux-kernel - Re: [RFC PATCH 03/12] netfs: Remove netfs_read

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Date:   Wed, 21 Jul 2021 15:00:43 -0400
From:   Jeff Layton <jlayton@...hat.com>
To:     David Howells <dhowells@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Mike Marshall <hubcap@...ibond.com>,
        David Wysochanski <dwysocha@...hat.com>,
        Shyam Prasad N <nspmangalore@...il.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-cachefs@...hat.com, linux-afs@...ts.infradead.org,
        linux-nfs@...r.kernel.org, linux-cifs@...r.kernel.org,
        ceph-devel@...r.kernel.org, v9fs-developer@...ts.sourceforge.net,
        devel@...ts.orangefs.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 03/12] netfs: Remove
 netfs_read_subrequest::transferred

On Wed, 2021-07-21 at 19:54 +0100, David Howells wrote:
> Jeff Layton <jlayton@...hat.com> wrote:
> 
> > The above two deltas seem like they should have been in patch #2.
> 
> Yeah.  Looks like at least partially so.
> 
> > > @@ -635,15 +625,8 @@ void netfs_subreq_terminated(struct netfs_read_subrequest *subreq,
> > >  		goto failed;
> > >  	}
> > >  
> > > -	if (WARN(transferred_or_error > subreq->len - subreq->transferred,
> > > -		 "Subreq overread: R%x[%x] %zd > %zu - %zu",
> > > -		 rreq->debug_id, subreq->debug_index,
> > > -		 transferred_or_error, subreq->len, subreq->transferred))
> > > -		transferred_or_error = subreq->len - subreq->transferred;
> > > -
> > >  	subreq->error = 0;
> > > -	subreq->transferred += transferred_or_error;
> > > -	if (subreq->transferred < subreq->len)
> > > +	if (iov_iter_count(&subreq->iter))
> > >  		goto incomplete;
> > >  
> > 
> > I must be missing it, but where does subreq->iter get advanced to the
> > end of the current read? If you're getting rid of subreq->transferred
> > then I think that has to happen above, no?
> 
> For afs, afs_req_issue_op() points fsreq->iter at the subrequest iterator and
> calls afs_fetch_data().  Thereafter, we wend our way to
> afs_deliver_fs_fetch_data() or yfs_deliver_fs_fetch_data() which set
> call->iter to point to that iterator and then call afs_extract_data() which
> passes it to rxrpc_kernel_recv_data(), which eventually passes it to
> skb_copy_datagram_iter(), which advances the iterator.
> 
> For the cache, the subrequest iterator is passed to the cache backend by
> netfs_read_from_cache().  This would be cachefiles_read() which calls
> vfs_iocb_iter_read() which I thought advances the iterator (leastways,
> filemap_read() keeps going until iov_iter_count() reaches 0 or some other stop
> condition occurs and doesn't thereafter call iov_iter_revert()).
> 

Ok, this will probably regress ceph then. We don't really have anything
to do with the subreq->iter at this point and this patch doesn't change
that. If you're going to make this change, it'd be cleaner to also fix
up ceph_netfs_issue_op to advance the iter at the same time.
-- 
Jeff Layton <jlayton@...hat.com>