[Crash-utility] Negative NR_WRITEBACK counter

[Buland Kumar Singh]

Hello Everyone,

I am analysing a kernel crash dump (vmcore) captured from RHEL-5
kernel version (2.6.18-371.4.1.el5) and found that the value of
"NR_WRITEBACK" counter is negative (-126).

$ rpm -q crash
crash-7.0.6-2.el6.x86_64

crash> sys | grep -e RELEASE -e MACHINE -e MEMORY
     RELEASE: 2.6.18-371.4.1.el5
     MACHINE: x86_64  (3000 Mhz)
      MEMORY: 31.5 GB

crash> kmem -z | grep  -e ZONE -e NR_WRITEBACK
NODE: 0  ZONE: 0  ADDR: ffff810000032000  NAME: "DMA"
       NR_WRITEBACK: 0
NODE: 0  ZONE: 1  ADDR: ffff810000032b00  NAME: "DMA32"
       NR_WRITEBACK: 0
NODE: 0  ZONE: 2  ADDR: ffff810000033600  NAME: "Normal"
       NR_WRITEBACK: -126          <<<<
NODE: 0  ZONE: 3  ADDR: ffff810000034100  NAME: "HighMem"

crash> kmem -V | grep -e NR_WRITEBACK
       NR_WRITEBACK: -126          <<<<

crash> vm_stat
vm_stat = $1 =
 {{
    counter = 1106459
  }, {
    counter = 2940354
  }, {
    counter = 6341366
  }, {
    counter = 301750
  }, {
    counter = 245858
  }, {
    counter = 438
  }, {
    counter = -126  // NR_WRITEBACK <<<<
  }, {
    counter = 0
  }, {
    counter = 0
  }, {
    counter = 19687071384
  }, {
    counter = 0
  }, {
    counter = 0
  }, {
    counter = 29247123
  }, {
    counter = 19687071384
  }, {
    counter = 0
  }}

As we're running a 64 bit kernel and the counters are signed long,
so this is very unlikely to be a counter overflow. I need pointers
and suggestions to determine the *cause* of negative counter from
vmcore.

Additional Information:

$ git show ce866b34ae1b7f1ce60234cf65855886ac7e7d30
[..]
diff --git a/drivers/base/node.c b/drivers/base/node.c
index 6fed520..a7b3dcb 100644
--- a/drivers/base/node.c
+++ b/drivers/base/node.c
@@ -49,9 +49,6 @@ static ssize_t node_read_meminfo(struct sys_device * dev,
char * buf)
        get_page_state_node(&ps, nid);
        __get_zone_counts(&active, &inactive, &free, NODE_DATA(nid));

-       /* Check for negative values in these approximate counters */
-       if ((long)ps.nr_writeback < 0)
-               ps.nr_writeback = 0;

        n = sprintf(buf, "\n"
                       "Node %d MemTotal:     %8lu kB\n"
[..]

Thank you !

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