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, 41 insertions, 1 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 135d6637afe5..a86e7b445800 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -7097,6 +7097,41 @@ int __mem_cgroup_charge(struct folio *folio, struct mm_struct *mm, gfp_t gfp)
 }
 
 /**
+ * mem_cgroup_hugetlb_try_charge - try to charge the memcg for a hugetlb folio
+ * @memcg: memcg to charge.
+ * @gfp: reclaim mode.
+ * @nr_pages: number of pages to charge.
+ *
+ * This function is called when allocating a huge page folio to determine if
+ * the memcg has the capacity for it. It does not commit the charge yet,
+ * as the hugetlb folio itself has not been obtained from the hugetlb pool.
+ *
+ * Once we have obtained the hugetlb folio, we can call
+ * mem_cgroup_commit_charge() to commit the charge. If we fail to obtain the
+ * folio, we should instead call mem_cgroup_cancel_charge() to undo the effect
+ * of try_charge().
+ *
+ * Returns 0 on success. Otherwise, an error code is returned.
+ */
+int mem_cgroup_hugetlb_try_charge(struct mem_cgroup *memcg, gfp_t gfp,
+			long nr_pages)
+{
+	/*
+	 * If hugetlb memcg charging is not enabled, do not fail hugetlb allocation,
+	 * but do not attempt to commit charge later (or cancel on error) either.
+	 */
+	if (mem_cgroup_disabled() || !memcg ||
+		!cgroup_subsys_on_dfl(memory_cgrp_subsys) ||
+		!(cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_HUGETLB_ACCOUNTING))
+		return -EOPNOTSUPP;
+
+	if (try_charge(memcg, gfp, nr_pages))
+		return -ENOMEM;
+
+	return 0;
+}
+
+/**
  * mem_cgroup_swapin_charge_folio - Charge a newly allocated folio for swapin.
  * @folio: folio to charge.
  * @mm: mm context of the victim
@@ -7365,7 +7400,12 @@ void mem_cgroup_migrate(struct folio *old, struct folio *new)
 		return;
 
 	memcg = folio_memcg(old);
-	VM_WARN_ON_ONCE_FOLIO(!memcg, old);
+	/*
+	 * Note that it is normal to see !memcg for a hugetlb folio.
+	 * For e.g, itt could have been allocated when memory_hugetlb_accounting
+	 * was not selected.
+	 */
+	VM_WARN_ON_ONCE_FOLIO(!folio_test_hugetlb(old) && !memcg, old);
 	if (!memcg)
 		return;