* [PATCH RESEND 1/2] loop: deprecate autoloading callback loop_probe()
2023-07-17 19:16 [PATCH RESEND 0/2] loop: fix regression from max_loop default value change Mauricio Faria de Oliveira
@ 2023-07-17 19:16 ` Mauricio Faria de Oliveira
2023-07-20 8:30 ` Christoph Hellwig
2023-07-17 19:16 ` [PATCH RESEND 2/2] loop: do not enforce max_loop hard limit by (new) default Mauricio Faria de Oliveira
1 sibling, 1 reply; 6+ messages in thread
From: Mauricio Faria de Oliveira @ 2023-07-17 19:16 UTC (permalink / raw)
To: Jens Axboe, linux-block; +Cc: Isaac J. Manjarres
The 'probe' callback in __register_blkdev() is only used
under the CONFIG_BLOCK_LEGACY_AUTOLOAD deprecation guard.
The loop_probe() function is only used for that callback,
so guard it too, accordingly.
See commit fbdee71bb5d8 ("block: deprecate autoloading based on dev_t").
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
---
drivers/block/loop.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 37511d2b2caf..7268ff71c92c 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -2093,6 +2093,7 @@ static void loop_remove(struct loop_device *lo)
put_disk(lo->lo_disk);
}
+#ifdef CONFIG_BLOCK_LEGACY_AUTOLOAD
static void loop_probe(dev_t dev)
{
int idx = MINOR(dev) >> part_shift;
@@ -2101,6 +2102,7 @@ static void loop_probe(dev_t dev)
return;
loop_add(idx);
}
+#endif
static int loop_control_remove(int idx)
{
@@ -2235,8 +2237,11 @@ static int __init loop_init(void)
if (err < 0)
goto err_out;
-
+#ifdef CONFIG_BLOCK_LEGACY_AUTOLOAD
if (__register_blkdev(LOOP_MAJOR, "loop", loop_probe)) {
+#else
+ if (register_blkdev(LOOP_MAJOR, "loop")) {
+#endif
err = -EIO;
goto misc_out;
}
--
2.39.2
^ permalink raw reply related [flat|nested] 6+ messages in thread* [PATCH RESEND 2/2] loop: do not enforce max_loop hard limit by (new) default
2023-07-17 19:16 [PATCH RESEND 0/2] loop: fix regression from max_loop default value change Mauricio Faria de Oliveira
2023-07-17 19:16 ` [PATCH RESEND 1/2] loop: deprecate autoloading callback loop_probe() Mauricio Faria de Oliveira
@ 2023-07-17 19:16 ` Mauricio Faria de Oliveira
2023-07-20 8:31 ` Christoph Hellwig
1 sibling, 1 reply; 6+ messages in thread
From: Mauricio Faria de Oliveira @ 2023-07-17 19:16 UTC (permalink / raw)
To: Jens Axboe, linux-block; +Cc: Isaac J. Manjarres
Problem:
The max_loop parameter is used for 2 different purposes:
1) initial number of loop devices to pre-create on init
2) maximum number of loop devices to add on access/open()
Historically, its default value (zero) caused 1) to create
non-zero number of devices (CONFIG_BLK_DEV_LOOP_MIN_COUNT),
and _no_ hard limit on 2) to add devices with autoloading.
However, the default value changed in commit 85c50197716c
("loop: Fix the max_loop commandline argument treatment
when it is set to 0") to CONFIG_BLK_DEV_LOOP_MIN_COUNT,
so that max_loop=0 does not pre-create devices at all.
That does improve 1), but unfortunately _breaks_ 2), as
default behavior *changed* from no-limit to hard-limit.
Example:
For example, this userspace code broke for N >= CONFIG,
if the user relied on the default value 0 for max_loop:
mknod("/dev/loopN")
open("/dev/loopN") // now fails with ENXIO
Though affected users may "fix" it with (loop.)max_loop=0,
this means to require a kernel parameter change on stable
kernel update (that commit Fixes: an old commit in stable).
Solution:
The original semantics for the default value in 2) can be
applied if the parameter is not set (ie, default behavior).
This still keeps the intended function in 1) and 2) if set,
and that commit's intended improvement in 1) if max_loop=0.
Before 85c50197716c:
- default: 1) CONFIG devices 2) no limit
- max_loop=0: 1) CONFIG devices 2) no limit
- max_loop=X: 1) X devices 2) X limit
After 85c50197716c:
- default: 1) CONFIG devices 2) CONFIG limit (*)
- max_loop=0: 1) 0 devices (*) 2) no limit
- max_loop=X: 1) X devices 2) X limit
This commit:
- default: 1) CONFIG devices 2) no limit (*)
- max_loop=0: 1) 0 devices 2) no limit
- max_loop=X: 1) X devices 2) X limit
Future:
The issue/regression from that commit only affects code
under the CONFIG_BLOCK_LEGACY_AUTOLOAD deprecation guard,
thus the fix too is contained under it.
Once that deprecated functionality/code is removed, the
purpose 2) of max_loop (hard limit) is no longer in use,
so the module parameter description can be changed then.
Tests:
Linux 6.5-rc2
CONFIG_BLK_DEV_LOOP_MIN_COUNT=8
CONFIG_BLOCK_LEGACY_AUTOLOAD=y
- default (original)
# ls -1 /dev/loop*
/dev/loop-control
/dev/loop0
...
/dev/loop7
# ./test-loop
open: /dev/loop8: No such device or address
- default (patched)
# ls -1 /dev/loop*
/dev/loop-control
/dev/loop0
...
/dev/loop7
# ./test-loop
#
- max_loop=0 (original & patched):
# ls -1 /dev/loop*
/dev/loop-control
# ./test-loop
#
- max_loop=8 (original & patched):
# ls -1 /dev/loop*
/dev/loop-control
/dev/loop0
...
/dev/loop7
# ./test-loop
open: /dev/loop8: No such device or address
- max_loop=0 (patched; CONFIG_BLOCK_LEGACY_AUTOLOAD is not set)
# ls -1 /dev/loop*
/dev/loop-control
# ./test-loop
open: /dev/loop8: No such device or address
Fixes: 85c50197716c ("loop: Fix the max_loop commandline argument treatment when it is set to 0")
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
---
drivers/block/loop.c | 36 ++++++++++++++++++++++++++++++++++--
1 file changed, 34 insertions(+), 2 deletions(-)
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 7268ff71c92c..310e21889086 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1775,14 +1775,43 @@ static const struct block_device_operations lo_fops = {
/*
* If max_loop is specified, create that many devices upfront.
* This also becomes a hard limit. If max_loop is not specified,
+ * the default isn't a hard limit (as before commit 85c50197716c
+ * changed the default value from 0 for max_loop=0 reasons), just
* create CONFIG_BLK_DEV_LOOP_MIN_COUNT loop devices at module
* init time. Loop devices can be requested on-demand with the
* /dev/loop-control interface, or be instantiated by accessing
* a 'dead' device node.
*/
static int max_loop = CONFIG_BLK_DEV_LOOP_MIN_COUNT;
-module_param(max_loop, int, 0444);
+
+#ifdef CONFIG_BLOCK_LEGACY_AUTOLOAD
+static bool max_loop_specified;
+
+static int max_loop_param_set_int(const char *val,
+ const struct kernel_param *kp)
+{
+ int ret;
+
+ ret = param_set_int(val, kp);
+ if (ret < 0)
+ return ret;
+
+ max_loop_specified = true;
+ return 0;
+}
+
+static const struct kernel_param_ops max_loop_param_ops = {
+ .set = max_loop_param_set_int,
+ .get = param_get_int,
+};
+
+module_param_cb(max_loop, &max_loop_param_ops, &max_loop, 0444);
MODULE_PARM_DESC(max_loop, "Maximum number of loop devices");
+#else
+module_param(max_loop, int, 0444);
+MODULE_PARM_DESC(max_loop, "Initial number of loop devices");
+#endif
+
module_param(max_part, int, 0444);
MODULE_PARM_DESC(max_part, "Maximum number of partitions per loop device");
@@ -2098,7 +2127,7 @@ static void loop_probe(dev_t dev)
{
int idx = MINOR(dev) >> part_shift;
- if (max_loop && idx >= max_loop)
+ if (max_loop_specified && max_loop && idx >= max_loop)
return;
loop_add(idx);
}
@@ -2286,6 +2315,9 @@ module_exit(loop_exit);
static int __init max_loop_setup(char *str)
{
max_loop = simple_strtol(str, NULL, 0);
+#ifdef CONFIG_BLOCK_LEGACY_AUTOLOAD
+ max_loop_specified = true;
+#endif
return 1;
}
--
2.39.2
^ permalink raw reply related [flat|nested] 6+ messages in thread