char_dev: extend dynamic allocation of majors into a higher range

We've run into problems with running out of dynamicly assign char
device majors particullarly on automated test systems with
all-yes-configs. Roughly 40 dynamic assignments can be made with such
kernels at this time while space is reserved for only 20.

Currently, the kernel only prints a warning when dynamic allocation
overflows the reserved region. And when this happens drivers that have
fixed assignments can randomly fail depending on the order of
initialization of other drivers. Thus, adding a new char device can cause
unexpected failures in completely unrelated parts of the kernel.

This patch solves the problem by extending dynamic major number
allocations down from 511 once the 234-254 region fills up. Fixed
majors already exist above 255 so the infrastructure to support
high number majors is already in place. The patch reserves an
additional 128 major numbers which should hopefully last us a while.

Kernels that don't require more than 20 dynamic majors assigned (which
is pretty typical) should not be affected by this change.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Linus Walleij <linus.walleij@linaro.org>
Link: https://lkml.org/lkml/2017/6/4/107
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/admin-guide/devices.txt b/Documentation/admin-guide/devices.txt
index 6b71852dadc277c638310beb63d89064024e7d5b..4ec843123cc3bd1f206aa8b2315b3acad11b1a88 100644 (file)
--- a/Documentation/admin-guide/devices.txt
+++ b/Documentation/admin-guide/devices.txt
@@ -3081,3 +3081,8 @@
                  1 = /dev/osd1         Second OSD Device
                  ...
                  255 = /dev/osd255     256th OSD Device
+
+ 384-511 char  RESERVED FOR DYNAMIC ASSIGNMENT
+               Character devices that request a dynamic allocation of major
+               number will take numbers starting from 511 and downward,
+               once the 234-254 range is full.

diff --git a/fs/char_dev.c b/fs/char_dev.c
index fb8507f521b265ba8feba385154cc908eb812f44..c9d18362e89de0a060f96ae8e41cc05543c9dc6f 100644 (file)
--- a/fs/char_dev.c
+++ b/fs/char_dev.c
@@ -59,6 +59,29 @@ void chrdev_show(struct seq_file *f, off_t offset)
 
 #endif /* CONFIG_PROC_FS */
 
+static int find_dynamic_major(void)
+{
+       int i;
+       struct char_device_struct *cd;
+
+       for (i = ARRAY_SIZE(chrdevs)-1; i > CHRDEV_MAJOR_DYN_END; i--) {
+               if (chrdevs[i] == NULL)
+                       return i;
+       }
+
+       for (i = CHRDEV_MAJOR_DYN_EXT_START;
+            i > CHRDEV_MAJOR_DYN_EXT_END; i--) {
+               for (cd = chrdevs[major_to_index(i)]; cd; cd = cd->next)
+                       if (cd->major == i)
+                               break;
+
+               if (cd == NULL || cd->major != i)
+                       return i;
+       }
+
+       return -EBUSY;
+}
+
 /*
  * Register a single major with a specified minor range.
  *
@@ -84,22 +107,14 @@ __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 (chrdevs[i] == NULL)
-                               break;
-               }
-
-               if (i < CHRDEV_MAJOR_DYN_END)
-                       pr_warn("CHRDEV \"%s\" major number %d goes below the dynamic allocation range\n",
-                               name, i);
-
-               if (i == 0) {
-                       ret = -EBUSY;
+               ret = find_dynamic_major();
+               if (ret < 0) {
+                       pr_err("CHRDEV \"%s\" dynamic allocation region is full\n",
+                              name);
                        goto out;
                }
-               major = i;
+               major = ret;
        }
 
        cd->major = major;

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 7b5d6816542b7f5af171d5680961aeb03184707b..1773dcf1e3180b19a1c14d81e287ef07739efaff 100644 (file)
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2473,6 +2473,10 @@ static inline void bd_unlink_disk_holder(struct block_device *bdev,
 #define CHRDEV_MAJOR_HASH_SIZE 255
 /* Marks the bottom of the first segment of free char majors */
 #define CHRDEV_MAJOR_DYN_END 234
+/* Marks the top and bottom of the second segment of free char majors */
+#define CHRDEV_MAJOR_DYN_EXT_START 511
+#define CHRDEV_MAJOR_DYN_EXT_END 384
+
 extern int alloc_chrdev_region(dev_t *, unsigned, unsigned, const char *);
 extern int register_chrdev_region(dev_t, unsigned, const char *);
 extern int __register_chrdev(unsigned int major, unsigned int baseminor,