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From:   David Howells <dhowells@redhat.com>
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To:     Jeff Layton <jlayton@redhat.com>
Cc:     dhowells@redhat.com, linux-fsdevel@vger.kernel.org,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
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        David Wysochanski <dwysocha@redhat.com>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
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Subject: Re: [RFC PATCH 03/12] netfs: Remove netfs_read_subrequest::transferred
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Date:   Wed, 21 Jul 2021 19:54:52 +0100
Message-ID: <298117.1626893692@warthog.procyon.org.uk>
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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_su=
brequest *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 =3D subreq->len - subreq->transferred;
> > -
> >  	subreq->error =3D 0;
> > -	subreq->transferred +=3D 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() whic=
h
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()).

David