[bug report] memcg: multi-memcg percpu charge cache - fix 2

[Dan Carpenter <dan.carpenter@linaro.org>]

Hello Shakeel Butt,

Commit 1db4ee9862f9 ("memcg: multi-memcg percpu charge cache - fix
2") from Apr 25, 2025 (linux-next), leads to the following Smatch
static checker warning:

	mm/memcontrol.c:1959 refill_stock()
	error: uninitialized symbol 'stock_pages'.

mm/memcontrol.c
    1907 static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
    1908 {
    1909         struct memcg_stock_pcp *stock;
    1910         struct mem_cgroup *cached;
    1911         uint8_t stock_pages;
                         ^^^^^^^^^^^

    1912         unsigned long flags;
    1913         bool success = false;
    1914         int empty_slot = -1;
    1915         int i;
    1916 
    1917         /*
    1918          * For now limit MEMCG_CHARGE_BATCH to 127 and less. In future if we
    1919          * decide to increase it more than 127 then we will need more careful
    1920          * handling of nr_pages[] in struct memcg_stock_pcp.
    1921          */
    1922         BUILD_BUG_ON(MEMCG_CHARGE_BATCH > S8_MAX);
    1923 
    1924         VM_WARN_ON_ONCE(mem_cgroup_is_root(memcg));
    1925 
    1926         if (nr_pages > MEMCG_CHARGE_BATCH ||
    1927             !local_trylock_irqsave(&memcg_stock.stock_lock, flags)) {
    1928                 /*
    1929                  * In case of larger than batch refill or unlikely failure to
    1930                  * lock the percpu stock_lock, uncharge memcg directly.
    1931                  */
    1932                 memcg_uncharge(memcg, nr_pages);
    1933                 return;
    1934         }
    1935 
    1936         stock = this_cpu_ptr(&memcg_stock);
    1937         for (i = 0; i < NR_MEMCG_STOCK; ++i) {
    1938                 cached = READ_ONCE(stock->cached[i]);
    1939                 if (!cached && empty_slot == -1)
    1940                         empty_slot = i;
    1941                 if (memcg == READ_ONCE(stock->cached[i])) {
    1942                         stock_pages = READ_ONCE(stock->nr_pages[i]) + nr_pages;
    1943                         WRITE_ONCE(stock->nr_pages[i], stock_pages);
    1944                         if (stock_pages > MEMCG_CHARGE_BATCH)
    1945                                 drain_stock(stock, i);
    1946                         success = true;
                                 ^^^^^^^^^^^^^^
When stock_pages is initialized then success is true.

    1947                         break;
    1948                 }
    1949         }
    1950 
    1951         if (!success) {
                     ^^^^^^^^
success is false.

    1952                 i = empty_slot;
    1953                 if (i == -1) {
    1954                         i = get_random_u32_below(NR_MEMCG_STOCK);
    1955                         drain_stock(stock, i);
    1956                 }
    1957                 css_get(&memcg->css);
    1958                 WRITE_ONCE(stock->cached[i], memcg);
--> 1959                 WRITE_ONCE(stock->nr_pages[i], stock_pages);
                                                        ^^^^^^^^^^^
This is always uninitialized at this point.  Probably on your test system
you are automatically initializing stack variables to zero.

    1960         }
    1961 
    1962         local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
    1963 }

regards,
dan carpenter