[PATCH 13/15] netfs: zero the tail of a short DIO/unbuffered read

[Paulo Alcantara <pc@manguebit.org>]

The buffered read collector zero-fills the tail of a short read that
stops below the inode's i_size (netfs_clear_unread()), so a read that
races an extending write still returns the expected number of bytes.
The non-buffered collector path does no such thing: it just records
how much was transferred.

Add the same zero-fill for the non-buffered case, gated on
NETFS_SREQ_CLEAR_TAIL: a subreq's source sets that flag when a short
result from it is known to be safe to treat as a hole, as opposed to
NETFS_SREQ_HIT_EOF, which means the read genuinely ran off the end of
the file and should be reported short as-is. Only CLEAR_TAIL should
zero-fill here; a real EOF must stay a real short read.

No source currently sets CLEAR_TAIL on an unbuffered/DIO subrequest,
so this is inert on its own -- a following change teaches cifs to set
it in the one case that needs it.

Fixes: e2d46f2ec332 ("netfs: Change the read result collector to only use one work item")
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org
---
 fs/netfs/read_collect.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/fs/netfs/read_collect.c b/fs/netfs/read_collect.c
index a94197ef0181..c5bfaf2c6f25 100644
--- a/fs/netfs/read_collect.c
+++ b/fs/netfs/read_collect.c
@@ -33,6 +33,22 @@ static void netfs_clear_unread(struct netfs_io_subrequest *subreq)
 		__set_bit(NETFS_SREQ_HIT_EOF, &subreq->flags);
 }
 
+static void netfs_clear_unread_dio(struct netfs_io_subrequest *subreq)
+{
+	uoff_t pos = subreq->start + subreq->transferred;
+	struct netfs_io_request *rreq = subreq->rreq;
+	size_t fill;
+
+	if (pos >= rreq->i_size)
+		return;
+
+	fill = min_t(uoff_t, rreq->i_size - pos,
+		     subreq->len - subreq->transferred);
+
+	netfs_reset_iter(subreq);
+	subreq->transferred += iov_iter_zero(fill, &subreq->io_iter);
+}
+
 /*
  * Cancel the copy-to-cache mark on a folio.
  */
@@ -311,6 +327,14 @@ static void netfs_collect_read_results(struct netfs_io_request *rreq)
 			    test_bit(NETFS_SREQ_HIT_EOF, &front->flags))
 				netfs_read_unlock_folios(rreq, &notes);
 		} else {
+			if (!(notes & HIT_PENDING) &&
+			    front->error == 0 &&
+			    transferred < front->len &&
+			    test_bit(NETFS_SREQ_CLEAR_TAIL, &front->flags)) {
+				netfs_clear_unread_dio(front);
+				transferred = front->transferred;
+				trace_netfs_sreq(front, netfs_sreq_trace_clear);
+			}
 			stream->collected_to = front->start + transferred;
 			rreq->collected_to = stream->collected_to;
 		}
-- 
2.55.0