cgroups: Redefine what "unlimited" means wrt memory limits

Since kernel 3.12 (commit 34ff8dc08956098563989d8599840b130be81252 in
linux-stable.git in particular) the value for 'unlimited' in cgroup
memory limits changed from LLONG_MAX to ULLONG_MAX.  Due to rather
unfortunate choice of our VIR_DOMAIN_MEMORY_PARAM_UNLIMITED constant
(which we transfer as an unsigned long long in Kibibytes), we ended up
with the situation described below (applies to x86_64):

 - 2^64-1 (ULLONG_MAX) -- "unlimited" in kernel = 3.12

 - 2^63-1 (LLONG_MAX) -- "unlimited" in kernel < 3.12
 - 2^63-1024 -- our PARAM_UNLIMITED scaled to Bytes

 - 2^53-1 -- our PARAM_UNLIMITED unscaled (in Kibibytes)

This means that when any number within (2^63-1, 2^64-1] is read from
memory cgroup, we are transferring that number instead of "unlimited".
Unfortunately, changing VIR_DOMAIN_MEMORY_PARAM_UNLIMITED would break
ABI compatibility and thus we have to resort to a different solution.

With this patch every value greater than PARAM_UNLIMITED means
"unlimited".  Even though this may seem misleading, we are already in
such unclear situation when running 3.12 kernel with memory limits set
to 2^63.

One example showing most of the problems at once (with kernel 3.12.2):
 # virsh memtune asdf --hard-limit 9007199254740991 --swap-hard-limit -1
 # echo 12345678901234567890 >\
/sys/fs/cgroup/memory/machine/asdf.libvirt-qemu/memory.soft_limit_in_bytes
 # virsh memtune asdf
 hard_limit     : 18014398509481983
 soft_limit     : 12056327051986884
 swap_hard_limit: 18014398509481983
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/util/vircgroup.c

@@ -1957,12 +1957,19 @@ int
 virCgroupGetMemoryHardLimit(virCgroupPtr group, unsigned long long *kb)
 {
     long long unsigned int limit_in_bytes;
-    int ret;
-    ret = virCgroupGetValueU64(group,
-                               VIR_CGROUP_CONTROLLER_MEMORY,
-                               "memory.limit_in_bytes", &limit_in_bytes);
-    if (ret == 0)
-        *kb = limit_in_bytes >> 10;
+    int ret = -1;
+
+    if (virCgroupGetValueU64(group,
+                             VIR_CGROUP_CONTROLLER_MEMORY,
+                             "memory.limit_in_bytes", &limit_in_bytes) < 0)
+        goto cleanup;
+
+    *kb = limit_in_bytes >> 10;
+    if (*kb > VIR_DOMAIN_MEMORY_PARAM_UNLIMITED)
+        *kb = VIR_DOMAIN_MEMORY_PARAM_UNLIMITED;
+
+    ret = 0;
+ cleanup:
     return ret;
 }
 
@@ -2012,12 +2019,19 @@ int
 virCgroupGetMemorySoftLimit(virCgroupPtr group, unsigned long long *kb)
 {
     long long unsigned int limit_in_bytes;
-    int ret;
-    ret = virCgroupGetValueU64(group,
-                               VIR_CGROUP_CONTROLLER_MEMORY,
-                               "memory.soft_limit_in_bytes", &limit_in_bytes);
-    if (ret == 0)
-        *kb = limit_in_bytes >> 10;
+    int ret = -1;
+
+    if (virCgroupGetValueU64(group,
+                             VIR_CGROUP_CONTROLLER_MEMORY,
+                             "memory.soft_limit_in_bytes", &limit_in_bytes) < 0)
+        goto cleanup;
+
+    *kb = limit_in_bytes >> 10;
+    if (*kb > VIR_DOMAIN_MEMORY_PARAM_UNLIMITED)
+        *kb = VIR_DOMAIN_MEMORY_PARAM_UNLIMITED;
+
+    ret = 0;
+ cleanup:
     return ret;
 }
 
@@ -2067,12 +2081,19 @@ int
 virCgroupGetMemSwapHardLimit(virCgroupPtr group, unsigned long long *kb)
 {
     long long unsigned int limit_in_bytes;
-    int ret;
-    ret = virCgroupGetValueU64(group,
-                               VIR_CGROUP_CONTROLLER_MEMORY,
-                               "memory.memsw.limit_in_bytes", &limit_in_bytes);
-    if (ret == 0)
-        *kb = limit_in_bytes >> 10;
+    int ret = -1;
+
+    if (virCgroupGetValueU64(group,
+                             VIR_CGROUP_CONTROLLER_MEMORY,
+                             "memory.memsw.limit_in_bytes", &limit_in_bytes) < 0)
+        goto cleanup;
+
+    *kb = limit_in_bytes >> 10;
+    if (*kb > VIR_DOMAIN_MEMORY_PARAM_UNLIMITED)
+        *kb = VIR_DOMAIN_MEMORY_PARAM_UNLIMITED;
+
+    ret = 0;
+ cleanup:
     return ret;
 }