hugetlb: memcg: account hugetlb-backed memory in memory controller

Currently, hugetlb memory usage is not acounted for in the memory
controller, which could lead to memory overprotection for cgroups with
hugetlb-backed memory.  This has been observed in our production system.

For instance, here is one of our usecases: suppose there are two 32G
containers.  The machine is booted with hugetlb_cma=6G, and each container
may or may not use up to 3 gigantic page, depending on the workload within
it.  The rest is anon, cache, slab, etc.  We can set the hugetlb cgroup
limit of each cgroup to 3G to enforce hugetlb fairness.  But it is very
difficult to configure memory.max to keep overall consumption, including
anon, cache, slab etc.  fair.

What we have had to resort to is to constantly poll hugetlb usage and
readjust memory.max.  Similar procedure is done to other memory limits
(memory.low for e.g).  However, this is rather cumbersome and buggy. 
Furthermore, when there is a delay in memory limits correction, (for e.g
when hugetlb usage changes within consecutive runs of the userspace
agent), the system could be in an over/underprotected state.

This patch rectifies this issue by charging the memcg when the hugetlb
folio is utilized, and uncharging when the folio is freed (analogous to
the hugetlb controller).  Note that we do not charge when the folio is
allocated to the hugetlb pool, because at this point it is not owned by
any memcg.

Some caveats to consider:
  * This feature is only available on cgroup v2.
  * There is no hugetlb pool management involved in the memory
    controller. As stated above, hugetlb folios are only charged towards
    the memory controller when it is used. Host overcommit management
    has to consider it when configuring hard limits.
  * Failure to charge towards the memcg results in SIGBUS. This could
    happen even if the hugetlb pool still has pages (but the cgroup
    limit is hit and reclaim attempt fails).
  * When this feature is enabled, hugetlb pages contribute to memory
    reclaim protection. low, min limits tuning must take into account
    hugetlb memory.
  * Hugetlb pages utilized while this option is not selected will not
    be tracked by the memory controller (even if cgroup v2 is remounted
    later on).

Link: https://lkml.kernel.org/r/20231006184629.155543-4-nphamcs@gmail.com
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Frank van der Linden <fvdl@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tejun heo <tj@kernel.org>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 29 insertions, 0 deletions

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 622a7f28db1f..606b2e0eac4b 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -210,6 +210,35 @@ cgroup v2 currently supports the following mount options.
         relying on the original semantics (e.g. specifying bogusly
         high 'bypass' protection values at higher tree levels).
 
+  memory_hugetlb_accounting
+        Count HugeTLB memory usage towards the cgroup's overall
+        memory usage for the memory controller (for the purpose of
+        statistics reporting and memory protetion). This is a new
+        behavior that could regress existing setups, so it must be
+        explicitly opted in with this mount option.
+
+        A few caveats to keep in mind:
+
+        * There is no HugeTLB pool management involved in the memory
+          controller. The pre-allocated pool does not belong to anyone.
+          Specifically, when a new HugeTLB folio is allocated to
+          the pool, it is not accounted for from the perspective of the
+          memory controller. It is only charged to a cgroup when it is
+          actually used (for e.g at page fault time). Host memory
+          overcommit management has to consider this when configuring
+          hard limits. In general, HugeTLB pool management should be
+          done via other mechanisms (such as the HugeTLB controller).
+        * Failure to charge a HugeTLB folio to the memory controller
+          results in SIGBUS. This could happen even if the HugeTLB pool
+          still has pages available (but the cgroup limit is hit and
+          reclaim attempt fails).
+        * Charging HugeTLB memory towards the memory controller affects
+          memory protection and reclaim dynamics. Any userspace tuning
+          (of low, min limits for e.g) needs to take this into account.
+        * HugeTLB pages utilized while this option is not selected
+          will not be tracked by the memory controller (even if cgroup
+          v2 is remounted later on).
+
 
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