[1/2] vircgroupv2: Fix VM startup when legacy cgroups are defined

Message ID ad2f5b20929e510abc944c0bac4f2a1c3fa0d01c.1569532456.git.crobinso@redhat.com
State Accepted
Commit b5290a6e6e3c7af47969f88693fbdc6ff331dbb3
Headers show
Series
  • Fix cgroupv2 issue on Fedora 31
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Commit Message

Cole Robinson Sept. 26, 2019, 9:18 p.m.
On Fedora 31, starting a 'mock' build alters /proc/$pid/cgroup,
probably due to usage of systemd-nspawn.

Before:
$ cat /proc/self/cgroup
0::/user.slice/user-1000.slice/...

After:
$ cat /proc/self/cgroup
1:name=systemd:/
0::/user.slice/user-1000.slice/...

The cgroupv2 code mishandles that first line in the second case, which
causes VM startup to fail with: Unable to read from
'/sys/fs/cgroup/machine/cgroup.controllers': No such file or directory

The kernel docs[1] say that the cgroupv2 path will always start with
'0::', which in the code here controllers="". Only set the v2 placement
path when we see that cgroup file entry.

[1] https://www.kernel.org/doc/html/v5.3/admin-guide/cgroup-v2.html#processes

https://bugzilla.redhat.com/show_bug.cgi?id=1751120

Signed-off-by: Cole Robinson <crobinso@redhat.com>

---
 src/util/vircgroupv2.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

-- 
2.23.0

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Comments

Pavel Hrdina Sept. 27, 2019, 9:03 a.m. | #1
On Thu, Sep 26, 2019 at 05:18:40PM -0400, Cole Robinson wrote:
> On Fedora 31, starting a 'mock' build alters /proc/$pid/cgroup,

> probably due to usage of systemd-nspawn.

> 

> Before:

> $ cat /proc/self/cgroup

> 0::/user.slice/user-1000.slice/...

> 

> After:

> $ cat /proc/self/cgroup

> 1:name=systemd:/

> 0::/user.slice/user-1000.slice/...

> 

> The cgroupv2 code mishandles that first line in the second case, which

> causes VM startup to fail with: Unable to read from

> '/sys/fs/cgroup/machine/cgroup.controllers': No such file or directory

> 

> The kernel docs[1] say that the cgroupv2 path will always start with

> '0::', which in the code here controllers="". Only set the v2 placement

> path when we see that cgroup file entry.

> 

> [1] https://www.kernel.org/doc/html/v5.3/admin-guide/cgroup-v2.html#processes

> 

> https://bugzilla.redhat.com/show_bug.cgi?id=1751120

> 

> Signed-off-by: Cole Robinson <crobinso@redhat.com>

> ---

>  src/util/vircgroupv2.c | 6 +++++-

>  1 file changed, 5 insertions(+), 1 deletion(-)

> 

> diff --git a/src/util/vircgroupv2.c b/src/util/vircgroupv2.c

> index 0663c67190..0cb20e4896 100644

> --- a/src/util/vircgroupv2.c

> +++ b/src/util/vircgroupv2.c

> @@ -194,12 +194,16 @@ virCgroupV2DetectMounts(virCgroupPtr group,

>  static int

>  virCgroupV2DetectPlacement(virCgroupPtr group,

>                             const char *path,

> -                           const char *controllers ATTRIBUTE_UNUSED,

> +                           const char *controllers,

>                             const char *selfpath)

>  {

>      if (group->unified.placement)

>          return 0;

>  

> +    /* controllers="" indicates the cgroupv2 controller path */

> +    if (STRNEQ_NULLABLE(controllers, ""))

> +        return 0;


Fixes the issue reported by several users and looks reasonable.

I would use /* controllers == "" ...*/ to mach the following comment
and there is no need to use STRNEQ_NULLABLE as the caller already
checks if !controllers.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
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Patch

diff --git a/src/util/vircgroupv2.c b/src/util/vircgroupv2.c
index 0663c67190..0cb20e4896 100644
--- a/src/util/vircgroupv2.c
+++ b/src/util/vircgroupv2.c
@@ -194,12 +194,16 @@  virCgroupV2DetectMounts(virCgroupPtr group,
 static int
 virCgroupV2DetectPlacement(virCgroupPtr group,
                            const char *path,
-                           const char *controllers ATTRIBUTE_UNUSED,
+                           const char *controllers,
                            const char *selfpath)
 {
     if (group->unified.placement)
         return 0;
 
+    /* controllers="" indicates the cgroupv2 controller path */
+    if (STRNEQ_NULLABLE(controllers, ""))
+        return 0;
+
     /*
      * selfpath == "/" + path="" -> "/"
      * selfpath == "/libvirt.service" + path == "" -> "/libvirt.service"