diff options
| author | Shakeel Butt <shakeel.butt@linux.dev> | 2026-05-21 15:37:51 -0700 |
|---|---|---|
| committer | Andrew Morton <akpm@linux-foundation.org> | 2026-05-28 20:50:33 -0700 |
| commit | c0cafe24d3f6534294c4b2bc2d47734ff7cbd313 (patch) | |
| tree | 6f86b52abaaf0b6993be8fea21dc5018b7715929 | |
| parent | b4aea43cd37afad714b5684fe9fdfcb0e78dba26 (diff) | |
memcg: use round-robin victim selection in refill_stock
Harry Yoo reported that get_random_u32_below() is not safe to call in the
nmi context and memcg charge draining can happen in nmi context.
More specifically get_random_u32_below() is neither reentrant- nor
NMI-safe: it acquires a per-cpu local_lock via local_lock_irqsave() on the
batched_entropy_u32 state. An NMI that lands on a CPU mid-update of the
ChaCha batch state and recurses into the random subsystem would corrupt
that state. The memcg_stock local_trylock prevents re-entry on the percpu
stock itself, but cannot protect an unrelated subsystem's per-cpu lock.
Replace the random pick with a per-cpu round-robin counter stored in
memcg_stock_pcp and serialized by the same local_trylock that already
guards cached[] and nr_pages[]. No atomics, no random calls, no extra
locks needed.
Link: https://lore.kernel.org/20260521223751.3794625-1-shakeel.butt@linux.dev
Fixes: f735eebe55f8f ("memcg: multi-memcg percpu charge cache")
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reported-by: Harry Yoo <harry@kernel.org>
Closes: https://lore.kernel.org/4e20f643-6983-4b6e-b12d-c6c4eb20ae0c@kernel.org/
Acked-by: Harry Yoo (Oracle) <harry@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
| -rw-r--r-- | mm/memcontrol.c | 5 |
1 files changed, 4 insertions, 1 deletions
diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 177732fef010..1a4fd2504bcd 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -2011,6 +2011,7 @@ struct memcg_stock_pcp { struct work_struct work; unsigned long flags; + uint8_t drain_idx; }; static DEFINE_PER_CPU_ALIGNED(struct memcg_stock_pcp, memcg_stock) = { @@ -2194,7 +2195,9 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages) if (!success) { i = empty_slot; if (i == -1) { - i = get_random_u32_below(NR_MEMCG_STOCK); + i = stock->drain_idx++; + if (stock->drain_idx == NR_MEMCG_STOCK) + stock->drain_idx = 0; drain_stock(stock, i); } css_get(&memcg->css); |