linux-kernel - Re: swap_info_get: Bad swap offset entry 0200f8a7

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Date:   Fri, 20 Oct 2017 18:07:43 -0700 (PDT)
From:   Christian Kujau <lists@...dbynature.de>
To:     huang ying <huang.ying.caritas@...il.com>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Nitin Gupta <ngupta@...are.org>,
        Robert Schelander <rschelander@....at>
Subject: Re: swap_info_get: Bad swap offset entry 0200f8a7

On Fri, 20 Oct 2017, huang ying wrote:
> >       4 May  < Linux version 4.11.2-1-ARCH
> >       4 Jun  < Linux version 4.11.3-1-ARCH
> >       7 Jul  < Linux version 4.11.9-1-ARCH
> >       4 Aug  < Linux version 4.12.8-2-ARCH
> >      24 Sep  < Linux version 4.12.13-1-ARCH
> >     158 Oct  < Linux version 4.13.5-1-ARCH
> 
> So you have never seen this before 4.11 like 4.10?

Unfortunately the kernel logs for that machine only go back until May 
2017 and I cannot tell if that hasn't happened before. I've seen these 
messages appear since then but didn't bother much. But as it now happens 
more frequently, I thought I should mention this to the list.

> Which operations will trigger this error messages?

I'm not able to reproduce it at will, but I suspect that memory pressure 
triggers these messages. The machine in question is an Lenovo Ideapad S10 
notebook running 24x7 and is equipped with 1 GB of RAM. Two Java processes 
are basically using up all the memory, so usually it tooks like this:

========================================
$ free -m
       total   used  free   shared  buff/cache available
Mem:     994    866    67       1           60        20
Swap:    760    437   322

$ zramctl 
NAME       ALGORITHM DISKSIZE  DATA COMPR TOTAL STREAMS MOUNTPOINT
/dev/zram0 lz4         248.7M  247M 92.3M 97.4M       2 [SWAP]
========================================

I just assumed the message is triggered when the system is really low on 
memory and maybe zram is too slow to provide the memory requested. But 
that's just my layman's assumption :-) For example, today's message was 
emitted during the night:


Oct 20 01:26:18 len kernel: [638973.207849] \
   swap_info_get: Bad swap offset entry 0200f8a7


And here are the sysstat numbers for that time frame:


$ sar -r -s 00:00 -e 02:00
Linux 4.13.5-1-ARCH (len)       10/20/2017      _i686_  (2 CPU)
12:00:01 AM kbmemfree kbmemused  %memused kbbuffers  kbcached  kbcommit   %commit  kbactive   kbinact   kbdirty
12:10:06 AM     70076    948404     93.12         4     19004   1556176     86.58    376608    379408       220
12:20:02 AM     80488    937992     92.10         4    180404   1563952     87.01    380184    327736      5568
12:30:03 AM     83296    935184     91.82         4    137260   1569776     87.34    329512    330000       280
12:40:03 AM     65188    953292     93.60         4     21156   1571048     87.41    386644    389820      1144
12:50:03 AM     67512    950968     93.37         4     33452   1570628     87.38    378936    381580      1304
01:00:07 AM     65520    952960     93.57         4     24996   1573180     87.53    385396    386152       904
01:10:03 AM     66956    951524     93.43         4     35520   1572696     87.50    379548    379364       172
01:20:02 AM     67440    951040     93.38         4     88736   1569864     87.34    381764    370472      7080
01:30:03 AM     70048    948432     93.12         4     29212   1572504     87.49    383516    381900      1832
01:40:04 AM     71532    946948     92.98         4     29220   1570096     87.35    380120    380284      1000
01:50:03 AM     65828    952652     93.54         4     34408   1570604     87.38    381040    381028      1604
Average:        70353    948127     93.09         4     57579   1569139     87.30    376661    371613      1919


 == If that is unreadable, here it is again: https://paste.debian.net/991927/

> Is it possible for you to check
> whether the error exists for normal swap device (not ZRAM)?

I have "normal" (but encrpted) swap configured but with a lower priority:

cat /proc/swaps 
Filename        Type            Size    Used    Priority
/dev/dm-0       partition       524284  194348  0
/dev/zram0      partition       254616  253536  32767

I shall disable the zram device and disable encryption too and will report 
back if the message appears again.

> 32bit or 64bit kernel do you use?

I'm using an i686 kernel for this Atom N270 processor (with HT enabled).

Thanks for your response,
Christian.
-- 
BOFH excuse #403:

Sysadmin didn't hear pager go off due to loud music from bar-room speakers.