[PATCH 1/2] blk-throttle: avoid ilog2(0) in calculate_bytes_allowed()

[Tao Cui]

From: Tao Cui <cuitao@xxxxxxxxxx>

__tg_update_carryover() can call calculate_bytes_allowed() with a zero
jiffy_elapsed right after a slice starts. The overflow guard

    if (ilog2(bps_limit) + ilog2(jiffy_elapsed) - ilog2(HZ) > 62)

relies on ilog2(0) == -1 (fls64(0) - 1) to stay below the threshold so
that the subsequent mul_u64_u64_div_u64(bps, 0, HZ) == 0 is reached.
That works, but the ilog2(0) dependency is non-obvious.

Add an explicit early return for jiffy_elapsed == 0, which is equivalent
(mul_u64_u64_div_u64(bps_limit, 0, HZ) == 0) and removes the reliance on
ilog2(0).

No behavior change.

Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
---
 block/blk-throttle.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index ffc3b70065d4..f37911abefdd 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -603,6 +603,10 @@ static unsigned int calculate_io_allowed(u32 iops_limit,
 
 static u64 calculate_bytes_allowed(u64 bps_limit, unsigned long jiffy_elapsed)
 {
+	/* 0 elapsed => 0 bytes allowed; also avoids ilog2(0) below. */
+	if (!jiffy_elapsed)
+		return 0;
+
 	/*
 	 * Can result be wider than 64 bits?
 	 * We check against 62, not 64, due to ilog2 truncation.
-- 
2.43.0