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

[Liqueur Librazy <im@librazy.org>]

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.