RHEL Engineering is moving the tracking of its product development work on RHEL 6 through RHEL 9 to Red Hat Jira (issues.redhat.com). If you're a Red Hat customer, please continue to file support cases via the Red Hat customer portal. If you're not, please head to the "RHEL project" in Red Hat Jira and file new tickets here. Individual Bugzilla bugs in the statuses "NEW", "ASSIGNED", and "POST" are being migrated throughout September 2023. Bugs of Red Hat partners with an assigned Engineering Partner Manager (EPM) are migrated in late September as per pre-agreed dates. Bugs against components "kernel", "kernel-rt", and "kpatch" are only migrated if still in "NEW" or "ASSIGNED". If you cannot log in to RH Jira, please consult article #7032570. That failing, please send an e-mail to the RH Jira admins at rh-issues@redhat.com to troubleshoot your issue as a user management inquiry. The email creates a ServiceNow ticket with Red Hat. Individual Bugzilla bugs that are migrated will be moved to status "CLOSED", resolution "MIGRATED", and set with "MigratedToJIRA" in "Keywords". The link to the successor Jira issue will be found under "Links", have a little "two-footprint" icon next to it, and direct you to the "RHEL project" in Red Hat Jira (issue links are of type "https://issues.redhat.com/browse/RHEL-XXXX", where "X" is a digit). This same link will be available in a blue banner at the top of the page informing you that that bug has been migrated.
Bug 1240494 - The system won't bypass the cache even enable the auto_dump_bypass_cache in qemu.conf
Summary: The system won't bypass the cache even enable the auto_dump_bypass_cache in q...
Keywords:
Status: CLOSED NOTABUG
Alias: None
Product: Red Hat Enterprise Linux 7
Classification: Red Hat
Component: libvirt
Version: 7.2
Hardware: ppc64le
OS: Linux
medium
medium
Target Milestone: rc
: ---
Assignee: Andrea Bolognani
QA Contact: Virtualization Bugs
URL:
Whiteboard:
Depends On:
Blocks:
TreeView+ depends on / blocked
 
Reported: 2015-07-07 06:14 UTC by Dan Zheng
Modified: 2017-04-07 10:42 UTC (History)
7 users (show)

Fixed In Version:
Doc Type: If docs needed, set a value
Doc Text:
Clone Of:
Environment:
Last Closed: 2017-04-07 10:42:14 UTC
Target Upstream Version:
Embargoed:

Attachments (Terms of Use)
Program showing the effect of O_DIRECT on /proc/*/fdinfo/* (1.69 KB, text/plain)
2017-04-07 10:41 UTC, Andrea Bolognani 		no flags Details
View All

Description Dan Zheng 2015-07-07 06:14:46 UTC 
Description of problem:
The system won't bypass the cache even enable the auto_dump_bypass_cache in qemu.conf. This problem does not exist on x86.

Version-Release number of selected component (if applicable):
libvirt-1.2.17-1.el7.ppc64le
qemu-kvm-rhev-2.3.0-5.el7.ppc64le
kernel-3.10.0-282.el7.ppc64le



How reproducible:
100%

Steps to Reproduce:
1. Set auto_dump_bypass_cache = 1 in qemu.conf
# cat /etc/libvirt/qemu.conf
...
auto_dump_bypass_cache = 1
...

2. Restart libvirtd, # systemctl restart libvirtd
3. Edit XML with <on_crash>coredump-restart</on_crash>
4. Start guest.
5. Prepare command line on another terminal-2 with below command.
# cat /proc/$(lsof -w /var/lib/libvirt/qemu/dump/*|awk '/libvirt_i/{print $2}')/fdinfo/*1*
6. Within the guest, stop kdump service and make the guest crash.
# systemctl stop kdump
# echo c > /proc/sysrq-trigger
7. Before the guest finishes crashing, run the command on step 5 for many times on terminal-2

# cat /proc/$(lsof -w /var/lib/libvirt/qemu/dump/*|awk '/libvirt_i/{print $2}')/fdinfo/*1*
pos:        3822059520
flags:        0600001
...
pos:        3968860160
flags:        0600001
...
pos:        4107272192
flags:        0600001

Actual result:
In the flags in step 6 output, the third value is '0' which stands for 'auto_dump_bypass_cache' is disabled. The system did not bypass the cache for auto dump with auto_dump_bypass_cache enabled.


Expect result:
In the flags in step 6 output, the third value is '4' which stands for 'auto_dump_bypass_cache' is enabled. The system should bypass the cache for auto dump with auto_dump_bypass_cache enabled.
Comment 3 Andrea Bolognani 2017-04-07 10:41:48 UTC 
Created attachment 1269673 [details]
Program showing the effect of O_DIRECT on /proc/*/fdinfo/*
Comment 4 Andrea Bolognani 2017-04-07 10:42:14 UTC 
(In reply to Dan Zheng from comment #0)
[...]
> # cat /proc/$(lsof -w /var/lib/libvirt/qemu/dump/*|awk '/libvirt_i/{print
> $2}')/fdinfo/*1*
> pos:        3822059520
> flags:        0600001
> ...
> pos:        3968860160
> flags:        0600001
> ...
> pos:        4107272192
> flags:        0600001
> 
> Actual result:
> In the flags in step 6 output, the third value is '0' which stands for
> 'auto_dump_bypass_cache' is disabled. The system did not bypass the cache
> for auto dump with auto_dump_bypass_cache enabled.
> 
> Expect result:
> In the flags in step 6 output, the third value is '4' which stands for
> 'auto_dump_bypass_cache' is enabled. The system should bypass the cache for
> auto dump with auto_dump_bypass_cache enabled.

The testing methodology is not entirely correct because it
doesn't account for the fact that the concrete value for eg.
O_DIRECT can be different depending on the architecture.

I've attached a simple C program that demostrates the issue:
when run on an x86_64 host, it will print

  $ ./direct whatever
  Without O_DIRECT:
  pos:	0
  flags:	0100001
  mnt_id:	62

  With O_DIRECT:
  pos:	0
  flags:	0140001
  mnt_id:	62

whereas on a ppc64le or aarch64 host it will print

  $ ./direct whatever
  Without O_DIRECT:
  pos:    0
  flags:  0400001
  mnt_id: 80

  With O_DIRECT:
  pos:    0
  flags:  0600001
  mnt_id: 80

So 0600001 is the expected output for ppc64le hosts, and
libvirt is behaving correctly. Closing as NOTABUG.
Note You need to log in before you can comment on or make changes to this bug.