[Crash-utility] Negative NR_WRITEBACK counter

[Buland Kumar Singh]

Thank you so much Dave


On 10 July 2014 22:47, Dave Anderson <anderson at redhat.com> wrote:

>
>
> ----- Original Message -----
> > 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.
>
> Since the vmcore is just a snapshot in time, I would think that it would
> be difficult/unlikely that you can determine *the cause* just from the
> system's state at the time of the dump.
>
> FWIW, I ran a quick test of ~200 sample vmcores, and find that negative
> values in the vm_stat[] array and per-zone statistics are not all that
> unusual, even on recent kernels.  (about 5% of the vmcores had one or
> more negative items).
>
> And if the counter value is required for some kind of VM-related decision
> or whatever, it would use one of the following functions, which return 0 in
> the case of negative values:
>
>   static inline unsigned long global_page_state(enum zone_stat_item item)
>   {
>           long x = atomic_long_read(&vm_stat[item]);
>   #ifdef CONFIG_SMP
>           if (x < 0)
>                   x = 0;
>   #endif
>           return x;
>   }
>
>   static inline unsigned long zone_page_state(struct zone *zone,
>                                           enum zone_stat_item item)
>   {
>           long x = atomic_long_read(&zone->vm_stat[item]);
>   #ifdef CONFIG_SMP
>           if (x < 0)
>                   x = 0;
>   #endif
>           return x;
>   }
>
> Dave
>
>
>
>
> >
> > 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 !
> >
> > --
> > BKS
> >
> > --
> > Crash-utility mailing list
> > Crash-utility at redhat.com
> > https://www.redhat.com/mailman/listinfo/crash-utility
>
> --
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-- 
BKS
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