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Subject[PATCH v2 2/3] mm-unstable: Multi-gen LRU: Avoid race in inc_min_seq()
From
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/

Fixes: d6c3af7d8a2b ("mm: multi-gen LRU: debugfs interface")
Cc: stable@vger.kernel.org
Cc: Yu Zhao <yuzhao@google.com>
Cc: Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---

Changes in v2:
- Add Fixes tag and cc stable

mm/vmscan.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 489a4fc7d9b1..6eecd291756c 100644
--- a/mm/vmscan.c
+++ b/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)) {
- 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;
}

/*
--
2.41.0.255.g8b1d071c50-goog
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