Multi-gen LRU: avoid race in inc_min_seq()

commit bb5e7f2 upstream.

inc_max_seq() will try to inc_min_seq() if nr_gens == MAX_NR_GENS. This
is because the generations are reused (the last oldest now empty
generation will become the next youngest generation).

inc_min_seq() is retried until successful, dropping the lru_lock
and yielding the CPU on each failure, and retaking the lock before
trying again:

        while (!inc_min_seq(lruvec, type, can_swap)) {
                spin_unlock_irq(&lruvec->lru_lock);
                cond_resched();
                spin_lock_irq(&lruvec->lru_lock);
        }

However, the initial condition that required incrementing the min_seq
(nr_gens == MAX_NR_GENS) is not retested. This can change by another
call to inc_max_seq() from run_aging() with force_scan=true from the
debugfs interface.

Since the eviction stalls when the nr_gens == MIN_NR_GENS, avoid
unnecessarily incrementing the min_seq by rechecking the number of
generations before each attempt.

This issue was uncovered in previous discussion on the list by Yu Zhao
and Aneesh Kumar [1].

[1] https://lore.kernel.org/linux-mm/CAOUHufbO7CaVm=xjEb1avDhHVvnC8pJmGyKcFf2iY_dpf+zR3w@mail.gmail.com/

Link: https://lkml.kernel.org/r/[email protected]
Fixes: d6c3af7 ("mm: multi-gen LRU: debugfs interface")
Signed-off-by: Kalesh Singh <[email protected]>
Tested-by: AngeloGioacchino Del Regno <[email protected]> [mediatek]
Tested-by: Charan Teja Kalla <[email protected]>
Cc: Yu Zhao <[email protected]>
Cc: Aneesh Kumar K V <[email protected]>
Cc: Barry Song <[email protected]>
Cc: Brian Geffon <[email protected]>
Cc: Jan Alexander Steffens (heftig) <[email protected]>
Cc: Lecopzer Chen <[email protected]>
Cc: Matthias Brugger <[email protected]>
Cc: Oleksandr Natalenko <[email protected]>
Cc: Qi Zheng <[email protected]>
Cc: Steven Barrett <[email protected]>
Cc: Suleiman Souhlal <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

mm/vmscan.c

@@ -4331,6 +4331,7 @@ static void inc_max_seq(struct lruvec *lruvec, bool can_swap, bool force_scan)
 	int type, zone;
 	struct lru_gen_struct *lrugen = &lruvec->lrugen;
 
+restart:
 	spin_lock_irq(&lruvec->lru_lock);
 
 	VM_WARN_ON_ONCE(!seq_is_valid(lruvec));
@@ -4341,11 +4342,12 @@ static void inc_max_seq(struct lruvec *lruvec, bool can_swap, bool force_scan)
 
 		VM_WARN_ON_ONCE(!force_scan && (type == LRU_GEN_FILE || can_swap));
 
-		while (!inc_min_seq(lruvec, type, can_swap)) {
-			spin_unlock_irq(&lruvec->lru_lock);
-			cond_resched();
-			spin_lock_irq(&lruvec->lru_lock);
-		}
+		if (inc_min_seq(lruvec, type, can_swap))
+			continue;
+
+		spin_unlock_irq(&lruvec->lru_lock);
+		cond_resched();
+		goto restart;
 	}
 
 	/*