This looks like a similar issue with:
https://bugzilla.redhat.com/show_bug.cgi?id=1644600

On the same VM, if you use latest RHEL-8.1 instead, is it still reproducible?

(In reply to Kairui Song from comment #2)
> This looks like a similar issue with:
> https://bugzilla.redhat.com/show_bug.cgi?id=1644600

For rhel7.6, with set-vmmemory can also result to vmcore-incomplete and similar console logs. Seems the same issue on rhel8.0(kexec-tools-2.0.15-21.el7_6.3.x86_64)

> 
> On the same VM, if you use latest RHEL-8.1 instead, is it still reproducible?
No, the issue does not exist on RHEL-8.1(20190523.0) with kexec-tools-2.0.19-3.el8.x86_64

Upstream: PG_offline essentially replaces PG_balloon.

However, in RHEL7, PG_balloon is still needed for other purposes: balloon compaction

We could

a) Backport b1123ea6d3b3d ("mm: balloon: use general non-lru movable page feature") and friends, to free up PG_balloon

b) Introduce a new MAPCOUNT value for PG_offline downstream, letting it co-exist with PG_balloon

c) Let it remain broken in RHEL7

I *guess* b) would be more feasible than a). I suspect that a) is quite involved.

(In reply to David Hildenbrand from comment #5)
> Upstream: PG_offline essentially replaces PG_balloon.
> 
> However, in RHEL7, PG_balloon is still needed for other purposes: balloon
> compaction
> 
> We could
> 
> a) Backport b1123ea6d3b3d ("mm: balloon: use general non-lru movable page
> feature") and friends, to free up PG_balloon
> 
> b) Introduce a new MAPCOUNT value for PG_offline downstream, letting it
> co-exist with PG_balloon
> 
> c) Let it remain broken in RHEL7
> 
> I *guess* b) would be more feasible than a). I suspect that a) is quite
> involved.

Thanks, I agree plan b is a feasible solution. I've cloned a bug for kernel fix, bz1718771, will you implement it for RHEL-7?

The kexec-tools backport should be pretty easy I assume.

I'll have a look at the 7.7? backport and let you know when I run into issues.

Testing with virtio-balloon without balloon compaction, not with Hyper-V, leaving that to the experts. To test with virtio-balloon, a special kernel build is required (CONFIG_BALLOON_COMPACTION=n).

-> Task info: https://brewweb.devel.redhat.com/taskinfo?taskID=22226003

[cloud-user@rhel7 ~]$ uname -a
Linux rhel7 3.10.0-1057.el7.test.x86_64 #1 SMP Tue Jun 18 07:21:55 EDT 2019 x86_64 x86_64 x86_64 GNU/Linux

[root@rhel7 cloud-user]# grep "BALLOON_COMPACTION" /boot/config-3.10.0-1057.el7. 
config-3.10.0-1057.el7.test.x86_64
[root@rhel7 cloud-user]# grep "BALLOON_COMPACTION" /boot/config-3.10.0-1057.el7.test.x86_64
# CONFIG_BALLOON_COMPACTION is not set



1. Start a guest with 8GB of memory, modified kernel and custom built "makedumpfile" installed.

[cloud-user@rhel7 ~]$ cat /proc/meminfo 
MemTotal:        8008712 kB
MemFree:         7683636 kB
MemAvailable:    7630212 kB
Buffers:            2088 kB

2. Inflate the balloon (notice that the crashkernel area also consumes memory)

[dhildenb@virtlab412 ~]$ echo "balloon 700" | sudo nc -U /var/tmp/monitor
QEMU 2.12.0 monitor - type 'help' for more information
(qemu) balloon 700
[dhildenb@virtlab412 ~]$ echo "info balloon" | sudo nc -U /var/tmp/monitor
QEMU 2.12.0 monitor - type 'help' for more information
(qemu) info balloon
balloon: actual=700

[cloud-user@rhel7 ~]$ cat /proc/meminfo 
MemTotal:         336904 kB
MemFree:          136264 kB
MemAvailable:      23600 kB
Buffers:             724 kB

3. Modify /etc/kdump.conf to display verbose information when dumping

-> core_collector makedumpfile -l --message-level 31 -d 31

4. Restart kdump

[guest] $ systemctl restart kdump

5. Trigger a kernel crash

[guest] $ echo 1 > /proc/sys/kernel/sysrq
[guest] $ echo c > /proc/sysrq-trigger


Guest restarts into kdump kernel and performs the dump. Being quick to capture the output:

Original pages  : 0x00000000001f7514
  Excluded pages   : 0x00000000001e9150
    Pages filled with zero  : 0x0000000000006bad
    Non-private cache pages : 0x000000000000370a
    Private cache pages     : 0x000000000000000f
    User process data pages : 0x0000000000002a10
    Free pages              : 0x000000000000807a
    Hwpoison pages          : 0x0000000000000000
    Offline pages           : 0x00000000001d4400
  Remaining pages  : 0x000000000000e3c4
  (The number of pages is reduced to 2%.)
Memory Hole     : 0x0000000000048aec
--------------------------------------------------
Total pages     : 0x0000000000240000


8008712 kB - 336904 kB = 7671808 == 1917952 pages == 0x1D4400 pages

-> All inflated pages (offline) got excluded.


Similar approach will also work for testing under Hyper-V (inflate the balloon differently - enable dynamic memory).

In contrast to RHEL8, this will *not* work with
- XEN balloon - XEN patch to mark pages offline is not included
- virtio-balloon (with CONFIG_BALLOON_COMPACTION=y) - Pages are marked PageOffline() and PageBalloon() -> kdump cannot handle this yet.

So this really is only to fix Hyper-V.

Hi David,

Thanks for the work! I've backported your "[PATCH] exclude pages that are logically offline". But to get the patch merged we need three acks and blocker flag for RHEL-7.7.

Can you also give devel_ack to the kernel bug? The dependency issue is because that bug is cloned which have a default dependency, I'll fix that.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2019:2134