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char_dev: extend dynamic allocation of majors into a higher range

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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>
This commit is contained in:
Logan Gunthorpe 2017-06-15 14:05:20 -06:00 committed by Greg Kroah-Hartman
parent f5ac3c49ff
commit a5d31a3f81
3 changed files with 37 additions and 13 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
Documentation/admin-guide/devices.txt
View File

@ -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.

41
fs/char_dev.c
View File

@ -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;

4
include/linux/fs.h
View File

@ -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,