On Wed 01-05-24 02:54:20, Breno Leitao wrote:
> A memcg pointer in the per-cpu stock can be accessed by drain_all_stock()
> and consume_stock() in parallel, causing a potential race.
>
> KCSAN shows this data-race clearly in the splat below:
>
> BUG: KCSAN: data-race in drain_all_stock.part.0 / try_charge_memcg
>
> write to 0xffff88903f8b0788 of 4 bytes by task 35901 on cpu 2:
> try_charge_memcg (mm/memcontrol.c:2323 mm/memcontrol.c:2746)
> __mem_cgroup_charge (mm/memcontrol.c:7287 mm/memcontrol.c:7301)
> do_anonymous_page (mm/memory.c:1054 mm/memory.c:4375 mm/memory.c:4433)
> __handle_mm_fault (mm/memory.c:3878 mm/memory.c:5300 mm/memory.c:5441)
> handle_mm_fault (mm/memory.c:5606)
> do_user_addr_fault (arch/x86/mm/fault.c:1363)
> exc_page_fault (./arch/x86/include/asm/irqflags.h:37
> ./arch/x86/include/asm/irqflags.h:72
> arch/x86/mm/fault.c:1513
> arch/x86/mm/fault.c:1563)
> asm_exc_page_fault (./arch/x86/include/asm/idtentry.h:623)
>
> read to 0xffff88903f8b0788 of 4 bytes by task 287 on cpu 27:
> drain_all_stock.part.0 (mm/memcontrol.c:2433)
> mem_cgroup_css_offline (mm/memcontrol.c:5398 mm/memcontrol.c:5687)
> css_killed_work_fn (kernel/cgroup/cgroup.c:5521 kernel/cgroup/cgroup.c:5794)
> process_one_work (kernel/workqueue.c:3254)
> worker_thread (kernel/workqueue.c:3329 kernel/workqueue.c:3416)
> kthread (kernel/kthread.c:388)
> ret_from_fork (arch/x86/kernel/process.c:147)
> ret_from_fork_asm (arch/x86/entry/entry_64.S:257)
>
> value changed: 0x00000014 -> 0x00000013
>
> This happens because drain_all_stock() is reading stock->nr_pages, while
> consume_stock() might be updating the same address, causing a potential
> data-race.
>
> Make the shared addresses bulletproof regarding to reads and writes,
> similarly to what stock->cached_objcg and stock->cached.
> Annotate all accesses to stock->nr_pages with READ_ONCE()/WRITE_ONCE().
>
> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
It is worth mentioning that the race is harmless.
> ---
> mm/memcontrol.c | 24 +++++++++++++++---------
> 1 file changed, 15 insertions(+), 9 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index fabce2b50c69..d3befe3b62fa 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2310,6 +2310,7 @@ static void memcg_account_kmem(struct mem_cgroup *memcg, int nr_pages)
> static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> {
> struct memcg_stock_pcp *stock;
> + unsigned int stock_pages;
> unsigned long flags;
> bool ret = false;
>
> @@ -2319,8 +2320,9 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> local_lock_irqsave(&memcg_stock.stock_lock, flags);
>
> stock = this_cpu_ptr(&memcg_stock);
> - if (memcg == READ_ONCE(stock->cached) && stock->nr_pages >= nr_pages) {
> - stock->nr_pages -= nr_pages;
> + stock_pages = READ_ONCE(stock->nr_pages);
> + if (memcg == READ_ONCE(stock->cached) && stock_pages >= nr_pages) {
> + WRITE_ONCE(stock->nr_pages, stock_pages - nr_pages);
> ret = true;
> }
>
> @@ -2334,16 +2336,18 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> */
> static void drain_stock(struct memcg_stock_pcp *stock)
> {
> + unsigned int stock_pages = READ_ONCE(stock->nr_pages);
> struct mem_cgroup *old = READ_ONCE(stock->cached);
>
> if (!old)
> return;
>
> - if (stock->nr_pages) {
> - page_counter_uncharge(&old->memory, stock->nr_pages);
> + if (stock_pages) {
> + page_counter_uncharge(&old->memory, stock_pages);
> if (do_memsw_account())
> - page_counter_uncharge(&old->memsw, stock->nr_pages);
> - stock->nr_pages = 0;
> + page_counter_uncharge(&old->memsw, stock_pages);
> +
> + WRITE_ONCE(stock->nr_pages, 0);
> }
>
> css_put(&old->css);
> @@ -2380,6 +2384,7 @@ static void drain_local_stock(struct work_struct *dummy)
> static void __refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> {
> struct memcg_stock_pcp *stock;
> + unsigned int stock_pages;
>
> stock = this_cpu_ptr(&memcg_stock);
> if (READ_ONCE(stock->cached) != memcg) { /* reset if necessary */
> @@ -2387,9 +2392,10 @@ static void __refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> css_get(&memcg->css);
> WRITE_ONCE(stock->cached, memcg);
> }
> - stock->nr_pages += nr_pages;
> + stock_pages = READ_ONCE(stock->nr_pages) + nr_pages;
> + WRITE_ONCE(stock->nr_pages, stock_pages);
>
> - if (stock->nr_pages > MEMCG_CHARGE_BATCH)
> + if (stock_pages > MEMCG_CHARGE_BATCH)
> drain_stock(stock);
> }
>
> @@ -2428,7 +2434,7 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
>
> rcu_read_lock();
> memcg = READ_ONCE(stock->cached);
> - if (memcg && stock->nr_pages &&
> + if (memcg && READ_ONCE(stock->nr_pages) &&
> mem_cgroup_is_descendant(memcg, root_memcg))
> flush = true;
> else if (obj_stock_flush_required(stock, root_memcg))
> --
> 2.43.0
--
Michal Hocko
SUSE Labs
