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SubjectRe: [RFC PATCH 03/12] netfs: Remove netfs_read_subrequest::transferred
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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>

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