netfs: zero the tail of a short DIO/unbuffered readgrafted

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>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Wilcox <willy@infradead.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

1 files changed, 62 insertions, 0 deletions

diff --git a/Documentation/admin-guide/vga-softcursor.rst b/Documentation/admin-guide/vga-softcursor.rst
new file mode 100644
index 000000000..f52175457
--- /dev/null
+++ b/Documentation/admin-guide/vga-softcursor.rst
@@ -0,0 +1,62 @@
+Software cursor for VGA
+=======================
+
+by Pavel Machek <pavel@atrey.karlin.mff.cuni.cz>
+and Martin Mares <mj@atrey.karlin.mff.cuni.cz>
+
+Linux now has some ability to manipulate cursor appearance.  Normally,
+you can set the size of hardware cursor.  You can now play a few new
+tricks: you can make your cursor look like a non-blinking red block,
+make it inverse background of the character it's over or to highlight
+that character and still choose whether the original hardware cursor
+should remain visible or not.  There may be other things I have never
+thought of.
+
+The cursor appearance is controlled by a ``<ESC>[?1;2;3c`` escape sequence
+where 1, 2 and 3 are parameters described below. If you omit any of them,
+they will default to zeroes.
+
+first Parameter
+	specifies cursor size::
+
+		0=default
+		1=invisible
+		2=underline,
+		...
+		8=full block
+		+ 16 if you want the software cursor to be applied
+		+ 32 if you want to always change the background color
+		+ 64 if you dislike having the background the same as the
+		     foreground.
+
+	Highlights are ignored for the last two flags.
+
+second parameter
+	selects character attribute bits you want to change
+	(by simply XORing them with the value of this parameter). On standard
+	VGA, the high four bits specify background and the low four the
+	foreground. In both groups, low three bits set color (as in normal
+	color codes used by the console) and the most significant one turns
+	on highlight (or sometimes blinking -- it depends on the configuration
+	of your VGA).
+
+third parameter
+	consists of character attribute bits you want to set.
+
+	Bit setting takes place before bit toggling, so you can simply clear a
+	bit by including it in both the set mask and the toggle mask.
+
+Examples
+--------
+
+To get normal blinking underline, use::
+
+	echo -e '\033[?2c'
+
+To get blinking block, use::
+
+	echo -e '\033[?6c'
+
+To get red non-blinking block, use::
+
+	echo -e '\033[?17;0;64c'