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Subject: Re: [RFC PATCH 03/12] netfs: Remove
 netfs_read_subrequest::transferred
From:   Jeff Layton <jlayton@redhat.com>
To:     David Howells <dhowells@redhat.com>
Cc:     linux-fsdevel@vger.kernel.org,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        Steve French <sfrench@samba.org>,
        Dominique Martinet <asmadeus@codewreck.org>,
        Mike Marshall <hubcap@omnibond.com>,
        David Wysochanski <dwysocha@redhat.com>,
        Shyam Prasad N <nspmangalore@gmail.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-cachefs@redhat.com, linux-afs@lists.infradead.org,
        linux-nfs@vger.kernel.org, linux-cifs@vger.kernel.org,
        ceph-devel@vger.kernel.org, v9fs-developer@lists.sourceforge.net,
        devel@lists.orangefs.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Date:   Wed, 21 Jul 2021 15:00:43 -0400
In-Reply-To: <298117.1626893692@warthog.procyon.org.uk>
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On Wed, 2021-07-21 at 19:54 +0100, David Howells wrote:
> Jeff Layton <jlayton@redhat.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@redhat.com>