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/[e-mail]
Signed-off-by: Nhat Pham <[e-mail]>
Acked-by: Johannes Weiner <[e-mail]>
Cc: Frank van der Linden <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Mike Kravetz <[e-mail]>
Cc: Muchun Song <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Cc: Roman Gushchin <[e-mail]>
Cc: Shakeel Butt <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Tejun heo <[e-mail]>
Cc: Yosry Ahmed <[e-mail]>
Cc: Zefan Li <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/admin-guide/cgroup-v2.rst

···
         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).
+
 
 Organizing Processes and Threads
 --------------------------------

include/linux/cgroup-defs.h

···
 	 * Enable recursive subtree protection
 	 */
 	CGRP_ROOT_MEMORY_RECURSIVE_PROT = (1 << 18),
+
+	/*
+	 * Enable hugetlb accounting for the memory controller.
+	 */
+	 CGRP_ROOT_MEMORY_HUGETLB_ACCOUNTING = (1 << 19),
 };
 
 /* cftype->flags */

include/linux/memcontrol.h

···
 	return __mem_cgroup_charge(folio, mm, gfp);
 }
 
+int mem_cgroup_hugetlb_try_charge(struct mem_cgroup *memcg, gfp_t gfp,
+		long nr_pages);
+
 int mem_cgroup_swapin_charge_folio(struct folio *folio, struct mm_struct *mm,
 				  gfp_t gfp, swp_entry_t entry);
 void mem_cgroup_swapin_uncharge_swap(swp_entry_t entry);
···
 
 static inline int mem_cgroup_charge(struct folio *folio,
 		struct mm_struct *mm, gfp_t gfp)
+{
+	return 0;
+}
+
+static inline int mem_cgroup_hugetlb_try_charge(struct mem_cgroup *memcg,
+		gfp_t gfp, long nr_pages)
 {
 	return 0;
 }

kernel/cgroup/cgroup.c

···
 	Opt_favordynmods,
 	Opt_memory_localevents,
 	Opt_memory_recursiveprot,
+	Opt_memory_hugetlb_accounting,
 	nr__cgroup2_params
 };
 
···
 	fsparam_flag("favordynmods",		Opt_favordynmods),
 	fsparam_flag("memory_localevents",	Opt_memory_localevents),
 	fsparam_flag("memory_recursiveprot",	Opt_memory_recursiveprot),
+	fsparam_flag("memory_hugetlb_accounting", Opt_memory_hugetlb_accounting),
 	{}
 };
 
···
 	case Opt_memory_recursiveprot:
 		ctx->flags |= CGRP_ROOT_MEMORY_RECURSIVE_PROT;
 		return 0;
+	case Opt_memory_hugetlb_accounting:
+		ctx->flags |= CGRP_ROOT_MEMORY_HUGETLB_ACCOUNTING;
+		return 0;
 	}
 	return -EINVAL;
 }
···
 			cgrp_dfl_root.flags |= CGRP_ROOT_MEMORY_RECURSIVE_PROT;
 		else
 			cgrp_dfl_root.flags &= ~CGRP_ROOT_MEMORY_RECURSIVE_PROT;
+
+		if (root_flags & CGRP_ROOT_MEMORY_HUGETLB_ACCOUNTING)
+			cgrp_dfl_root.flags |= CGRP_ROOT_MEMORY_HUGETLB_ACCOUNTING;
+		else
+			cgrp_dfl_root.flags &= ~CGRP_ROOT_MEMORY_HUGETLB_ACCOUNTING;
 	}
 }
 
···
 		seq_puts(seq, ",memory_localevents");
 	if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT)
 		seq_puts(seq, ",memory_recursiveprot");
+	if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_HUGETLB_ACCOUNTING)
+		seq_puts(seq, ",memory_hugetlb_accounting");
 	return 0;
 }
 
···
 			"nsdelegate\n"
 			"favordynmods\n"
 			"memory_localevents\n"
-			"memory_recursiveprot\n");
+			"memory_recursiveprot\n"
+			"memory_hugetlb_accounting\n");
 }
 static struct kobj_attribute cgroup_features_attr = __ATTR_RO(features);
 

mm/hugetlb.c

···
 				     pages_per_huge_page(h), folio);
 	hugetlb_cgroup_uncharge_folio_rsvd(hstate_index(h),
 					  pages_per_huge_page(h), folio);
+	mem_cgroup_uncharge(folio);
 	if (restore_reserve)
 		h->resv_huge_pages++;
 
···
 	struct hugepage_subpool *spool = subpool_vma(vma);
 	struct hstate *h = hstate_vma(vma);
 	struct folio *folio;
-	long map_chg, map_commit;
+	long map_chg, map_commit, nr_pages = pages_per_huge_page(h);
 	long gbl_chg;
-	int ret, idx;
+	int memcg_charge_ret, ret, idx;
 	struct hugetlb_cgroup *h_cg = NULL;
+	struct mem_cgroup *memcg;
 	bool deferred_reserve;
+	gfp_t gfp = htlb_alloc_mask(h) | __GFP_RETRY_MAYFAIL;
+
+	memcg = get_mem_cgroup_from_current();
+	memcg_charge_ret = mem_cgroup_hugetlb_try_charge(memcg, gfp, nr_pages);
+	if (memcg_charge_ret == -ENOMEM) {
+		mem_cgroup_put(memcg);
+		return ERR_PTR(-ENOMEM);
+	}
 
 	idx = hstate_index(h);
 	/*
···
 	 * code of zero indicates a reservation exists (no change).
 	 */
 	map_chg = gbl_chg = vma_needs_reservation(h, vma, addr);
-	if (map_chg < 0)
+	if (map_chg < 0) {
+		if (!memcg_charge_ret)
+			mem_cgroup_cancel_charge(memcg, nr_pages);
+		mem_cgroup_put(memcg);
 		return ERR_PTR(-ENOMEM);
+	}
 
 	/*
 	 * Processes that did not create the mapping will have no
···
 	 */
 	if (map_chg || avoid_reserve) {
 		gbl_chg = hugepage_subpool_get_pages(spool, 1);
-		if (gbl_chg < 0) {
-			vma_end_reservation(h, vma, addr);
-			return ERR_PTR(-ENOSPC);
-		}
+		if (gbl_chg < 0)
+			goto out_end_reservation;
 
 		/*
 		 * Even though there was no reservation in the region/reserve
···
 			hugetlb_cgroup_uncharge_folio_rsvd(hstate_index(h),
 					pages_per_huge_page(h), folio);
 	}
+
+	if (!memcg_charge_ret)
+		mem_cgroup_commit_charge(folio, memcg);
+	mem_cgroup_put(memcg);
+
 	return folio;
 
 out_uncharge_cgroup:
···
 out_subpool_put:
 	if (map_chg || avoid_reserve)
 		hugepage_subpool_put_pages(spool, 1);
+out_end_reservation:
 	vma_end_reservation(h, vma, addr);
+	if (!memcg_charge_ret)
+		mem_cgroup_cancel_charge(memcg, nr_pages);
+	mem_cgroup_put(memcg);
 	return ERR_PTR(-ENOSPC);
 }
 

mm/memcontrol.c

···
 }
 
 /**
+ * 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
···
 		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;
 

mm/migrate.c

···
 
 	folio_copy_owner(newfolio, folio);
 
-	if (!folio_test_hugetlb(folio))
-		mem_cgroup_migrate(folio, newfolio);
+	mem_cgroup_migrate(folio, newfolio);
 }
 EXPORT_SYMBOL(folio_migrate_flags);