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From: Johannes Weiner <hannes@cmpxchg.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Jingfeng Xie <xiejingfeng@linux.alibaba.com>,
	Suren Baghdasaryan <surenb@google.com>,
	linux-kernel@vger.kernel.org
Subject: [PATCH 2/2] psi: fix a division error in psi poll()
Date: Tue,  3 Dec 2019 13:35:24 -0500
Message-ID: <20191203183524.41378-3-hannes@cmpxchg.org> (raw)
In-Reply-To: <20191203183524.41378-1-hannes@cmpxchg.org>

The psi window size is a u64 an can be up to 10 seconds right now,
which exceeds the lower 32 bits of the variable. We currently use
div_u64 for it, which is meant only for 32-bit divisors. The result is
garbage pressure sampling values and even potential div0 crashes.

Use div64_u64.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
---
 kernel/sched/psi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 970db4686dd4..ce8f6748678a 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -482,7 +482,7 @@ static u64 window_update(struct psi_window *win, u64 now, u64 value)
 		u32 remaining;
 
 		remaining = win->size - elapsed;
-		growth += div_u64(win->prev_growth * remaining, win->size);
+		growth += div64_u64(win->prev_growth * remaining, win->size);
 	}
 
 	return growth;
-- 
2.24.0


  parent reply index

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-03 18:35 [PATCH 0/2] psi: two division fixes Johannes Weiner
2019-12-03 18:35 ` [PATCH 1/2] psi: fix sampling error and rare div0 crashes with cgroups and high uptime Johannes Weiner
2019-12-17 12:39   ` [tip: sched/urgent] sched/psi: Fix " tip-bot2 for Johannes Weiner
2019-12-03 18:35 ` Johannes Weiner [this message]
2019-12-17 12:39   ` [tip: sched/urgent] psi: Fix a division error in psi poll() tip-bot2 for Johannes Weiner
2019-12-11 15:43 ` [PATCH 0/2] psi: two division fixes Peter Zijlstra

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