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Re: [libvirt] [RFC][PATCH] lxc: fix for ns cgroups subsystem



Hi Serge and Daniel,

On Sat, May 9, 2009 at 4:03 AM, Serge E. Hallyn <serue us ibm com> wrote:
> Quoting Daniel P. Berrange (berrange redhat com):
>> On Fri, May 08, 2009 at 08:34:12AM -0500, Serge E. Hallyn wrote:
>> > Quoting Ryota Ozaki (ozaki ryota gmail com):
>> > > Hi Serge,
>> > >
>> > > On Fri, May 8, 2009 at 11:48 AM, Serge E. Hallyn <serue us ibm com> wrote:
>> > > > IIUC, the real problem is that src/cgroup.c assumes that the
>> > > > cgroup name should be $CGROUP_MOUNTPOINT/groupname.  But of
>> > > > course if the ns cgroup is enabled, then the unshare(CLONE_NEWNS)
>> > > > to create a new namespace in which to mount the new devpts
>> > > > locks the driver under $CGROUP_MOUNTPOINT/<pid_of_driver>/
>> > > > or somesuch.
>> > > >
>> > > > If this fixes the problem I have no objections, but it seems
>> > > > more fragile than perhaps trying to teach src/cgroup.c to
>> > > > consider it's current cgroup as a starting point.
>> > >
>> > > hmm, I don't know why the assumption is bad and how the approach
>> > > you are suggesting helps the ns problem.
>> >
>> > To be clear, the asssumption is that the driver starts in the
>> > root cgroup, i.e. it's pid is listed in $CGROUP_MOUNTPOINT/tasks.
>> > And that it can create $CGROUP_MOUNTPOINT/groupname and move
>> > itself into $CGROUP_MOUNTPOINT/groupname/tasks.
>> >
>> > So, the assumption is bad because when the driver does a
>> > unshare(CLONE_NEWNS), it gets moved into $CGROUP_MOUNTPOINT/X,
>> > and after that can only move itself into
>> > $CGROUP_MOUNTPOINT/X/groupname.
>> >
>> > Even with your patch, it's possible for the lxc driver to have
>> > been started under say $CGROUP_MOUNTPOINT/libvir or
>> > $CGROUP_MOUNTPOINT/<username> through libcgroup/PAM for instance,
>> > in which case your patch would be insufficient.
>>
>> Indeed, we can't assume we're in the root group at any time. Our
>> general plan is that libvirt itself uses 3 levels of cgroup
>> starting at the cgroup that libvirtd was placed in by the admin
>> of the host, then a per-driver group, then per-guest groups
>>
>>   $LIBVIRTD_ROOT_CGROUP
>>      |
>>      +- lxc
>>      |    |
>>      |    +- LXC-GUEST-1
>>      |    +- LXC-GUEST-2
>>      |    +- LXC-GUEST-3
>>      |    +- ...
>>      |
>>      +- qemu
>>           |
>>           +- QEMU-GUEST-1
>>           +- QEMU-GUEST-2
>>           +- QEMU-GUEST-3
>>           +- ...
>>
>> $LIBVIRTD_ROOT_CGROUP, may be the actaul root mount point for
>> the cgroup controller in question, or it may be a sub-directory
>> that the admin chose to put it in. Also, if running libvirtd
>> as a normal user, the admin may have created per-user account
>> cgroups and so libvirtd would end up in there. So we have to
>> be prepared for anything wrt initial libvirtd cgroup root.
>>
>> NB The code for putting QEMU in a cgroup isn't merged yet.
>
> That sounds good.  I just don't think the code currently does
> it.  To do the right thing, IIUC, virCgroupPathOfGroup() should
> parse the /proc/pid/cgroup contents of the 'controller' process,
> and insert that between the virCgroupGetMount(controller)
> result and the group name.
>
> Or something...
>
> (right?)
>
> thanks,
> -serge
>

OK I probably understood the problem and wrote a patch for that
according to the Serge's suggestion.

I expect this patch fixes the problem, however unfortunately
I've not tried it yet under the situation you are worrying and
just done regression tests. Because I don't know easy way to
produce the situation. If you know that, please let me know...

Thanks,
  ozaki-r

>From 713be8125b5cbd3b60919afe708f9c788206a572 Mon Sep 17 00:00:00 2001
From: Ryota Ozaki <ozaki ryota gmail com>
Date: Sun, 10 May 2009 08:32:41 +0900
Subject: [PATCH] cgroups: address libvirtd's bad assumption for
cgroups hierarchy

---
 src/cgroup.c |   78 ++++++++++++++++++++++++++++++++++++++++++++++++++++------
 1 files changed, 70 insertions(+), 8 deletions(-)

diff --git a/src/cgroup.c b/src/cgroup.c
index 50517e2..e02c37f 100644
--- a/src/cgroup.c
+++ b/src/cgroup.c
@@ -117,11 +117,60 @@ int virCgroupHaveSupport(void)
     return 0;
 }

+static int virCgroupGetCurrentPath(int pid, char **curpath)
+{
+    char buf[PATH_MAX+1024];
+    char *path = NULL;
+    int rc = 0;
+    int len;
+    int fd;
+    char *pos = NULL;
+
+    if (virAsprintf(&path, "/proc/%d/cgroup", pid) == -1) {
+        rc = -ENOMEM;
+        goto error;
+    }
+
+    fd = open(path, O_RDONLY);
+    if (fd < 0) {
+        DEBUG("Unable to open %s: %m", path);
+        rc = -errno;
+        goto error_open;
+    }
+
+    len = saferead(fd, buf, sizeof(buf));
+    if (len <= 0) {
+        rc = -errno;
+        goto error_saferead;
+    }
+    buf[len-1] = '¥0'; /* overwrite '¥n' */
+
+    pos = strstr(buf, ":/");
+    if (pos == NULL) {
+        rc = -1;
+        goto error_strstr;
+    }
+
+    pos += 2; /* skip ':/' */
+    if (virAsprintf(curpath, "%s", pos) == -1) {
+        rc = -ENOMEM;
+    }
+
+error_strstr:
+error_saferead:
+    close(fd);
+error_open:
+    VIR_FREE(path);
+error:
+    return rc;
+}
+
 static int virCgroupPathOfGroup(const char *group,
                                 const char *controller,
                                 char **path)
 {
     virCgroupPtr root = NULL;
+    char *curpath = NULL;
     int rc = 0;

     root = virCgroupGetMount(controller);
@@ -130,8 +179,14 @@ static int virCgroupPathOfGroup(const char *group,
         goto out;
     }

-    if (virAsprintf(path, "%s/%s", root->path, group) == -1)
+    rc = virCgroupGetCurrentPath(getpid(), &curpath);
+    if (rc < 0)
+        goto out;
+
+    if (virAsprintf(path, "%s/%s/%s", root->path, curpath, group) == -1)
         rc = -ENOMEM;
+
+    VIR_FREE(curpath);
 out:
     virCgroupFree(&root);

@@ -145,6 +200,7 @@ static int virCgroupPathOf(const char *grppath,
     virCgroupPtr root;
     int rc = 0;
     char *controller = NULL;
+    char *curpath = NULL;

     if (strchr(key, '.') == NULL)
         return -EINVAL;
@@ -158,8 +214,14 @@ static int virCgroupPathOf(const char *grppath,
         goto out;
     }

-    if (virAsprintf(path, "%s/%s/%s", root->path, grppath, key) == -1)
+    rc = virCgroupGetCurrentPath(getpid(), &curpath);
+    if (rc < 0)
+        goto out;
+
+    if (virAsprintf(path, "%s/%s/%s/%s", root->path, curpath,
grppath, key) == -1)
         rc = -ENOMEM;
+
+    VIR_FREE(curpath);
 out:
     virCgroupFree(&root);
     VIR_FREE(controller);
@@ -625,15 +687,14 @@ int virCgroupAddTask(virCgroupPtr group, pid_t pid)
     char *grppath = NULL;
     char *taskpath = NULL;
     char *pidstr = NULL;
+    virCgroupPtr root;

     for (i = 0; supported_controllers[i] != NULL; i++) {
-        rc = virCgroupPathOfGroup(group->path,
-                                  supported_controllers[i],
-                                  &grppath);
-        if (rc != 0)
-            goto done;
+        root = virCgroupGetMount(supported_controllers[i]);
+        if (root == NULL)
+            continue;

-        if (virAsprintf(&taskpath, "%s/tasks", grppath) == -1) {
+        if (virAsprintf(&taskpath, "%s/%s/tasks", root->path,
group->path) == -1) {
             rc = -ENOMEM;
             goto done;
         }
@@ -655,6 +716,7 @@ int virCgroupAddTask(virCgroupPtr group, pid_t pid)
         }

     done:
+        virCgroupFree(&root);
         VIR_FREE(grppath);
         VIR_FREE(taskpath);
         VIR_FREE(pidstr);
-- 
1.6.0.6


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