Re: [BUG] blk-throttle panic on 32bit machine after startup

Re: [BUG] blk-throttle panic on 32bit machine after startup

[Haoran Luo]

Pardon me for elaborating some of my opinions.

> I think this piece of code presumes all jiffies values are greater than
> 0, which is the initial value assigned when kzalloc-ing throtl_grp. It
> fails on 32-bit linux for the first 5 minutes after booting, since the
> jiffies value then will be less than 0.

Expressing the jiffies value as greater or less than 0 is a mistake of vagueness. I actually means that comparison in macro "time_after_eq", which written as below in "5.16-rc2" in "include/linux/jiffies.h" at around line 110.

	#define time_after_eq(a,b)	\
		(typecheck(unsigned long, a) && \
		 typecheck(unsigned long, b) && \
		 ((long)((a) - (b)) >= 0))

The "INITIAL_JIFFIES" which is defined as "((unsigned long)(unsigned int)-300*HZ)", converts to "((long)-300*HZ)" and is smaller than "((long)0)". And similarily "timer_after_eq(x, 0)" being evaluated to false holds for x ranged from `INITIAL_JIFFIES <= x <= MAX_LONG` on 32-bit linux.

The same thing will not happen for 64-bit linux, since "((unsigned long)(unsigned int)-300*HZ)" is evaluated to a value greater than zero in the macro above.

I actually cannot figure out a fix for this problem on 32-bit, if it presumes the jiffies value in "tg->slice_start[rw]" to be either "0" or jiffies "x" holding property of "time_after_eq(x, 0)".

Re: [BUG] blk-throttle panic on 32bit machine after startup

[Haoran Luo]

(Sorry for the garbled message due to my mistaken the configuration of mutt)

I'm the college of the reporter and I would like to provide more
information.

The code in 5.15-rc6 in "blk-throttle.c" around line 791 is written as
below:

	/*
	 * Previous slice has expired. We must have trimmed it after
	 * last
	 * bio dispatch. That means since start of last slice, we never
	 * used
	 * that bandwidth. Do try to make use of that bandwidth while
	 * giving
	 * credit.
	 */
	if (time_after_eq(start, tg->slice_start[rw]))
		tg->slice_start[rw] = start;

I think this piece of code presumes all jiffies values are greater than
0, which is the initial value assigned when kzalloc-ing throtl_grp. It
fails on 32-bit linux for the first 5 minutes after booting, since the
jiffies value then will be less than 0.

[BUG] blk-throttle panic on 32bit machine after startup

[Youfu Zhang]

Hi,

I ran into a kernel bug related to blk-throttle on CentOS 7 AltArch for i386.
Userspace programs may panic the kernel if they hit the I/O limit
within 5 minutes after startup.

Root cause:
1. jiffies was initialized to -300HZ during boot on 32bit machines
2. enable blkio cgroup hierarchy
   __DEVEL__sane_behavior for cgroup v1 or default hierarchy for cgroup v2
   EL7 kernel modified throtl_pd_init and always enable hierarchical throttling
3. enable & trigger blkio throttling within 5 minutes after startup
   bio propagated from child tg to parent
4. enter throtl_start_new_slice_with_credit
   if(time_after_eq(start, tg->slice_start[rw]))
   aka. time_after_eq(0xFFFxxxxx, 0) does not hold
   parent tg->slice_start[rw] was zero-initialized and not updated
5. enter throtl_trim_slice
   BUG_ON(time_before(tg->slice_end[rw], tg->slice_start[rw]))
   aka. time_before(0xFFFxxxxx, 0) triggers a panic

Reproducer: (tested on Alpine Linux x86 kernel 5.10.X)
#!/bin/sh
CGROUP_PATH="$(mktemp -d)"
mount -t cgroup2 none "$CGROUP_PATH"
echo +io >"$CGROUP_PATH/cgroup.subtree_control"
mkdir "$CGROUP_PATH/child"
echo "7:0 riops=2" >"$CGROUP_PATH/child/io.max"
echo 0 >"$CGROUP_PATH/child/cgroup.procs"
echo 3 >/proc/sys/vm/drop_caches
dd if=/dev/loop0 of=/dev/null count=3

Re: [BUG] blk-throttle panic on 32bit machine after startup

[Liqueur Librazy]

Hi,

Yet another colleague of the reporter here. I found that some 
precondition maybe not sound when tg->slice_end[rw] is initialized with 
0, which time_before(INITIAL_JIFFIES, 0) holds true in 32-bit Linux.

As in v5.15-rc6/block/blk-throttle.c

1. L833

	/* Determine if previously allocated or extended slice is complete or 
not */
	static bool throtl_slice_used(struct throtl_grp *tg, bool rw)
	{
		if (time_in_range(jiffies, tg->slice_start[rw], tg->slice_end[rw]))
			return false;

		return true;
	}

throtl_slice_used will always return true for a newly initialized slice.
This may be intentional behavior but not mentioned in comment.
(except when jiffies == 0, which is another topic: will 
time_in_range_open do better here?)

2. L791, in throtl_start_new_slice_with_credit

	/*
	 * Previous slice has expired. We must have trimmed it after last
	 * bio dispatch. That means since start of last slice, we never used
	 * that bandwidth. Do try to make use of that bandwidth while giving
	 * credit.
	 */
	if (time_after_eq(start, tg->slice_start[rw]))
		tg->slice_start[rw] = start;

As mentioned in my colleague Haoran Luo's reply, time_after_eq(start, 
tg->slice_start[rw]) is falsy when the jiffies had not wrapped around.
A easy solution is to add a check for tg->slice_start[rw] == 0, or we 
should initialize tg->slice_start[rw] and tg->slice_end[rw] with 
INITIAL_JIFFIES.

Re: [BUG] blk-throttle panic on 32bit machine after startup

[Tejun Heo]

On Mon, Oct 18, 2021 at 03:08:53PM +0800, Youfu Zhang wrote:
> Hi,
> 
> I ran into a kernel bug related to blk-throttle on CentOS 7 AltArch for i386.
> Userspace programs may panic the kernel if they hit the I/O limit
> within 5 minutes after startup.
> 
> Root cause:
> 1. jiffies was initialized to -300HZ during boot on 32bit machines
> 2. enable blkio cgroup hierarchy
>    __DEVEL__sane_behavior for cgroup v1 or default hierarchy for cgroup v2
>    EL7 kernel modified throtl_pd_init and always enable hierarchical throttling
> 3. enable & trigger blkio throttling within 5 minutes after startup
>    bio propagated from child tg to parent
> 4. enter throtl_start_new_slice_with_credit
>    if(time_after_eq(start, tg->slice_start[rw]))
>    aka. time_after_eq(0xFFFxxxxx, 0) does not hold
>    parent tg->slice_start[rw] was zero-initialized and not updated
> 5. enter throtl_trim_slice
>    BUG_ON(time_before(tg->slice_end[rw], tg->slice_start[rw]))
>    aka. time_before(0xFFFxxxxx, 0) triggers a panic

This doesn't reproduce on 5.14.

Thanks.

-- 
tejun

Re: [BUG] blk-throttle panic on 32bit machine after startup

[Youfu Zhang]

> This doesn't reproduce on 5.14.

I can reproduce this bug on 5.14 i386.
I ran the reproducer (slightly modified, sr0 instead of loop0, 11:0
instead of 8:0) on Debian installer live CD.
https://cdimage.debian.org/cdimage/daily-builds/daily/current/i386/iso-cd/debian-testing-i386-netinst.iso
I posted a screen recording at https://youtu.be/ULdoHizTi0k. Please check.

Thanks.