[PATCH 2/2] RFC: chrdev: allocate chardevs in all unused holes
From: Linus Walleij
Date: Fri Feb 19 2016 - 09:36:34 EST
This is a duct-tape-and-chewing-gum solution to the problem with
the major numbers running out when allocating major numbers
dynamically.
To avoid collissions in the major space, we supply a list of
"holes" that exist in the lower range of major numbers [0-254]
and pick numbers from there once the first dynamic range in
234-254 is used up.
This starts to produce INFO prints if we run down and start
allocating major numbers in the "holes" below major 234 so
we know what is going on.
It will also FAIL if we actually fill up all free major
numbers. This seems to me like the reasonable thing to do
since these numbers are, after all, reserved.
This also deletes the comment /* temporary */ which must be
one of the biggest lies ever.
This also updates the Documentation/devices.txt document to
reflect that all these numbers are used for dynamic assignment.
Reported-by: Ying Huang <ying.huang@xxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Alan Cox <alan@xxxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
---
What do I know of the "real" solution to this problem, I like
Gregs solution to have *only* dynamic allocation of major numbers
(give take some like tty's I guess) but cannot really test such
an approach properly. Take this as a discussion starting point.
Maybe Intel's test server with 20+ dynamic major devices is
something extremely unlikely-in-actual-reality virtualized or
whatnot, Ying it'd be nice if you could shed some light on this.
See: https://lkml.org/lkml/2016/2/14/124
---
Documentation/devices.txt | 22 ++++++++++++++--------
fs/char_dev.c | 39 ++++++++++++++++++++++++++++++++-------
2 files changed, 46 insertions(+), 15 deletions(-)
diff --git a/Documentation/devices.txt b/Documentation/devices.txt
index 0a3588a9798d..f1c441e085e9 100644
--- a/Documentation/devices.txt
+++ b/Documentation/devices.txt
@@ -278,6 +278,8 @@ Your cooperation is appreciated.
associated with block devices. The binding to the
loop devices is handled by mount(8) or losetup(8).
+ 8 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
+
8 block SCSI disk devices (0-15)
0 = /dev/sda First SCSI disk whole disk
16 = /dev/sdb Second SCSI disk whole disk
@@ -650,7 +652,7 @@ Your cooperation is appreciated.
2 = /dev/sbpcd2 Panasonic CD-ROM controller 0 unit 2
3 = /dev/sbpcd3 Panasonic CD-ROM controller 0 unit 3
- 26 char
+ 26 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
26 block Second Matsushita (Panasonic/SoundBlaster) CD-ROM
0 = /dev/sbpcd4 Panasonic CD-ROM controller 1 unit 0
@@ -893,7 +895,7 @@ Your cooperation is appreciated.
...
39 block
- 40 char
+ 40 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
40 block
@@ -1157,7 +1159,7 @@ Your cooperation is appreciated.
NAMING CONFLICT -- PROPOSED REVISED NAME /dev/rpda0 etc
- 60-63 char LOCAL/EXPERIMENTAL USE
+ 60-63 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
60-63 block LOCAL/EXPERIMENTAL USE
Allocated for local/experimental use. For devices not
@@ -1671,7 +1673,7 @@ Your cooperation is appreciated.
disks (see major number 3) except that the limit on
partitions is 15.
- 93 char
+ 93 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
93 block NAND Flash Translation Layer filesystem
0 = /dev/nftla First NFTL layer
@@ -1679,7 +1681,7 @@ Your cooperation is appreciated.
...
240 = /dev/nftlp 16th NTFL layer
- 94 char
+ 94 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
94 block IBM S/390 DASD block storage
0 = /dev/dasda First DASD device, major
@@ -1772,7 +1774,7 @@ Your cooperation is appreciated.
...
15 = /dev/amiraid/ar?p15 15th partition
-102 char
+102 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
102 block Compressed block device
0 = /dev/cbd/a First compressed block device, whole device
@@ -2057,7 +2059,7 @@ Your cooperation is appreciated.
1 = /dev/vnet1 2nd virtual network
...
-120-127 char LOCAL/EXPERIMENTAL USE
+120-127 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
120-127 block LOCAL/EXPERIMENTAL USE
Allocated for local/experimental use. For devices not
@@ -2371,7 +2373,7 @@ Your cooperation is appreciated.
1 = /dev/gfax1 GammaLink channel 1
...
-159 char RESERVED
+159 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
159 block RESERVED
@@ -2959,6 +2961,8 @@ Your cooperation is appreciated.
...
196 = /dev/dvb/adapter3/video0 first video decoder of fourth card
+213-215 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
+
216 char Bluetooth RFCOMM TTY devices
0 = /dev/rfcomm0 First Bluetooth RFCOMM TTY device
1 = /dev/rfcomm1 Second Bluetooth RFCOMM TTY device
@@ -3001,6 +3005,8 @@ Your cooperation is appreciated.
same interface. For interface documentation see
http://www.vmelinux.org/.
+222-223 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
+
224 char A2232 serial card
0 = /dev/ttyY0 First A2232 port
1 = /dev/ttyY1 Second A2232 port
diff --git a/fs/char_dev.c b/fs/char_dev.c
index 687471dc04a0..001b037f6b98 100644
--- a/fs/char_dev.c
+++ b/fs/char_dev.c
@@ -37,6 +37,15 @@ static struct char_device_struct {
struct cdev *cdev; /* will die */
} *chrdevs[CHRDEV_MAJOR_HASH_SIZE];
+/*
+ * Duct tape: free major numbers that can be used below the
+ * primary dynamic character major assignment range 234-254
+ */
+static int free_majors[] = {223, 222, 215, 214, 213, 159, 127,
+ 126, 125, 124, 123, 122, 121, 120,
+ 102, 94, 93, 63, 62, 61, 60, 40,
+ 26, 8 };
+
/* index in the above */
static inline int major_to_index(unsigned major)
{
@@ -84,21 +93,37 @@ __register_chrdev_region(unsigned int major, unsigned int baseminor,
mutex_lock(&chrdevs_lock);
- /* temporary */
if (major == 0) {
for (i = ARRAY_SIZE(chrdevs)-1; i > 0; i--) {
+ if (i < CHRDEV_MAJOR_DYN_END)
+ break;
if (chrdevs[i] == NULL)
break;
}
- if (i < CHRDEV_MAJOR_DYN_END)
- pr_warn("CHRDEV \"%s\" major number %d goes below the dynamic allocation range",
+ /*
+ * Out of the first dynamic range, OK try this duct-tape
+ * array of lower majors.
+ */
+ if (i < CHRDEV_MAJOR_DYN_END) {
+ int j;
+
+ for (j = 0; j < ARRAY_SIZE(free_majors); j++) {
+ if (chrdevs[free_majors[j]] == NULL) {
+ i = free_majors[j];
+ break;
+ }
+ }
+ if (j == ARRAY_SIZE(free_majors)) {
+ pr_warn("CHRDEV \"%s\" out of major numbers",
+ name);
+ ret = -EBUSY;
+ goto out;
+ }
+ pr_info("CHRDEV: \"%s\" using extended dynamic range for major number %d\n",
name, i);
-
- if (i == 0) {
- ret = -EBUSY;
- goto out;
}
+
major = i;
}
--
2.4.3