Occasionally, testing memcg's move_charge_at_immigrate on rc7 shows a flurry of hundreds of warnings at kernel/res_counter.c:96, where res_counter_uncharge_locked() does WARN_ON(counter->usage < val). The first trace of each flurry implicates __mem_cgroup_cancel_charge() of mc.precharge, and an audit of mc.precharge handling points to mem_cgroup_move_charge_pte_range()'s THP handling in commit 12724850e806 ("memcg: avoid THP split in task migration"). Checking !mc.precharge is good everywhere else, when a single page is to be charged; but here the "mc.precharge -= HPAGE_PMD_NR" likely to follow, is liable to result in underflow (a lot can change since the precharge was estimated). Simply check against HPAGE_PMD_NR: there's probably a better alternative, trying precharge for more, splitting if unsuccessful; but this one-liner is safer for now - no kernel/res_counter.c:96 warnings seen in 26 hours.
Unified · Split
| @@ -5481,7 +5481,7 @@ static int mem_cgroup_move_charge_pte_range(pmd_t *pmd, · 5480 unmodified lines | |||
| 5481 | * part of thp split is not executed yet. | 5481 | * part of thp split is not executed yet. |
| 5482 | */ | 5482 | */ |
| 5483 | if (pmd_trans_huge_lock(pmd, vma) == 1) { | 5483 | if (pmd_trans_huge_lock(pmd, vma) == 1) { |
| 5484 | if (!mc.precharge) { | 5484 | if (mc.precharge < HPAGE_PMD_NR) { |
| 5485 | spin_unlock(&vma->vm_mm->page_table_lock); | 5485 | spin_unlock(&vma->vm_mm->page_table_lock); |
| 5486 | return 0; | 5486 | return 0; |
| 5487 | } | 5487 | } |