linux-kernel - [46/80] percpu: fix pcpu_last_unit

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Message-Id: <20100924162618.880742629@clark.site>
Date:	Fri, 24 Sep 2010 09:24:34 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Tejun Heo <tj@...nel.org>
Subject: [46/80] percpu: fix pcpu_last_unit_cpu

2.6.35-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Tejun Heo <tj@...nel.org>

commit 46b30ea9bc3698bc1d1e6fd726c9601d46fa0a91 upstream.

pcpu_first/last_unit_cpu are used to track which cpu has the first and
last units assigned.  This in turn is used to determine the span of a
chunk for man/unmap cache flushes and whether an address belongs to
the first chunk or not in per_cpu_ptr_to_phys().

When the number of possible CPUs isn't power of two, a chunk may
contain unassigned units towards the end of a chunk.  The logic to
determine pcpu_last_unit_cpu was incorrect when there was an unused
unit at the end of a chunk.  It failed to ignore the unused unit and
assigned the unused marker NR_CPUS to pcpu_last_unit_cpu.

This was discovered through kdump failure which was caused by
malfunctioning per_cpu_ptr_to_phys() on a kvm setup with 50 possible
CPUs by CAI Qian.

Signed-off-by: Tejun Heo <tj@...nel.org>
Reported-by: CAI Qian <caiqian@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 mm/percpu.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1413,9 +1413,9 @@ int __init pcpu_setup_first_chunk(const

 			if (pcpu_first_unit_cpu == NR_CPUS)
 				pcpu_first_unit_cpu = cpu;
+			pcpu_last_unit_cpu = cpu;
 		}
 	}
-	pcpu_last_unit_cpu = cpu;
 	pcpu_nr_units = unit;

 	for_each_possible_cpu(cpu)

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