github-mirror/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2026-06-08 14:42:37 +02:00
Code Issues Actions Packages Projects Releases Wiki Activity Graph

memcg: use round-robin victim selection in refill_stock

Browse Source 
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: f735eebe55 ("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>
This commit is contained in:
Shakeel Butt 2026-05-21 15:37:51 -07:00 committed by Andrew Morton
parent b4aea43cd3
commit c0cafe24d3
1 changed files with 4 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
mm/memcontrol.c
View File

@ -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);