netfs: Fix setting of transferred bytes with short DIO reads

netfs: Fix setting of transferred bytes with short DIO reads

A netfslib request comprises an ordered stream of subrequests that, when
doing an unbuffered/DIO read, are contiguous.  The subrequests may be
performed in parallel, but may not be fully completed.

For instance, if we try and make a 256KiB DIO read from a 3-byte file with
a 64KiB rsize and 256KiB bsize, netfslib will attempt to make a read of
256KiB, broken up into four 64KiB subreads, with the expectation that the
first will be short and the subsequent three be completely devoid - but we
do all four on the basis that the file may have been changed by a third
party.

The read-collection code, however, walks through all the subreqs and
advances the notion of how much data has been read in the stream to the
start of each subreq plus its amount transferred (which are 3, 0, 0, 0 for
the example above) - which gives an amount apparently read of 3*64KiB -
which is incorrect.

Fix the collection code to cut short the calculation of the transferred
amount with the first short subrequest in an unbuffered read; everything
beyond that must be ignored as there's a hole that cannot be filled.  This
applies both to shortness due to hitting the EOF and shortness due to an
error.

This is achieved by setting a flag on the request when we collect the first
short subrequest (collection is done in ascending order).

This can be tested by mounting a cifs volume with rsize=65536,bsize=262144
and doing a 256k DIO read of a very small file (e.g. 3 bytes).  read()
should return 3, not >3.

This problem came in when netfs_read_collection() set rreq->transferred to
stream->transferred, even for DIO.  Prior to that, netfs_rreq_assess_dio()
just went over the list and added up the subreqs till it met a short one -
but now the subreqs are discarded earlier.

Fixes: e2d46f2ec332 ("netfs: Change the read result collector to only use one work item")
Reported-by: Nicolas Baranger <nicolas.baranger@3xo.fr>
Closes: https://lore.kernel.org/all/10bec2430ed4df68bde10ed95295d093@3xo.fr/
Signed-off-by: "Paulo Alcantara (Red Hat)" <pc@manguebit.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/20250519090707.2848510-3-dhowells@redhat.com
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>