Multi-gen LRU: avoid race in inc_min_seq()

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/20230802025606.346758-2-kaleshsingh@google.com
Fixes: d6c3af7d ("mm: multi-gen LRU: debugfs interface")
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com> [mediatek]
Tested-by: Charan Teja Kalla <quic_charante@quicinc.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Jan Alexander Steffens (heftig) <heftig@archlinux.org>
Cc: Lecopzer Chen <lecopzer.chen@mediatek.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Oleksandr Natalenko <oleksandr@natalenko.name>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Steven Barrett <steven@liquorix.net>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmscan.c

@@ -4439,7 +4439,7 @@ static void inc_max_seq(struct lruvec *lruvec, bool can_swap, bool force_scan)
 	int prev, next;
 	int type, zone;
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
-
+restart:
 	spin_lock_irq(&lruvec->lru_lock);
 
 	VM_WARN_ON_ONCE(!seq_is_valid(lruvec));
@@ -4450,11 +4450,12 @@ 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)) {
+		if (inc_min_seq(lruvec, type, can_swap))
+			continue;
+
 		spin_unlock_irq(&lruvec->lru_lock);
 		cond_resched();
-			spin_lock_irq(&lruvec->lru_lock);
-		}
+		goto restart;
 	}
 
 	/*