Subject: Re: [PATCH v4 12/39] netfs: Add iov_iters to (sub)requests to describe various buffers

From: David Howells <[email-masked]>
Date: Tue, 19 Dec 2023 14:31:53 +0000

Jeff Layton <[email-masked]> wrote:

> > @@ -408,6 +417,10 @@ int netfs_write_begin(struct netfs_inode *ctx,
> >  	ractl._nr_pages = folio_nr_pages(folio);
> >  	netfs_rreq_expand(rreq, &ractl);
> >  
> > +	/* Set up the output buffer */
> > +	iov_iter_xarray(&rreq->iter, ITER_DEST, &mapping->i_pages,
> > +			rreq->start, rreq->len);
> 
> Should the above be ITER_SOURCE ?

No - we're in ->write_begin() and are prefetching.  If you look in the code,
there's a netfs_begin_read() call a few lines below.  The output buffer for
the read is the page we're going to write into.

Note that netfs_write_begin() should be considered deprecated as the whole
perform_write thing will get replaced.

> > @@ -88,6 +78,11 @@ static void netfs_read_from_server(struct netfs_io_request *rreq,
> >  				   struct netfs_io_subrequest *subreq)
> >  {
> >  	netfs_stat(&netfs_n_rh_download);
> > +	if (iov_iter_count(&subreq->io_iter) != subreq->len - subreq->transferred)
> > +		pr_warn("R=%08x[%u] ITER PRE-MISMATCH %zx != %zx-%zx %lx\n",
> > +			rreq->debug_id, subreq->debug_index,
> > +			iov_iter_count(&subreq->io_iter), subreq->len,
> > +			subreq->transferred, subreq->flags);
> 
> pr_warn is a bit alarmist, esp given the cryptic message.  Maybe demote
> this to INFO or DEBUG?
> 
> Does this indicate a bug in the client or that the server is sending us
> malformed frames?

Good question.  The network filesystem updated subreq->transferred to indicate
it had transferred X amount of data, but the iterator had been updated to
indicate Y amount of data was transferred.  They really ought to match as it
may otherwise indicate an underrun (and potential leakage of old data).
Overruns are less of a problem since the iterator would have to 'go negative'
as it were.

However, it might be better just to leave io_iter unchecked since we end up
resetting it anyway each time we reinvoke the ->issue_read() op.  It's always
possible that it will get copied and a different iterator get passed to the
network layer or cache fs - and so the change to the iterator then has to be
manually propagated just to avoid the warning.

David