memcg,thp: fix res_counter:96 regression

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.

HDHugh Dickins committed 14 years ago