regression 4.8+ commit 8ea1d2a (mm, frontswap: convert frontswap_enabled to static key) cause memory leak on swapon

[Christian Borntraeger]

Vlastimil,

with commit 8ea1d2a1985a7ae096e ("mm, frontswap: convert frontswap_enabled to static key")
kmemleak complains about a memory leak in swapon

unreferenced object 0x3e09ba56000 (size 32112640):
  comm "swapon", pid 7852, jiffies 4294968787 (age 1490.770s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<00000000003a2504>] __vmalloc_node_range+0x194/0x2d8
    [<00000000003a2918>] vzalloc+0x58/0x68
    [<00000000003b0af0>] SyS_swapon+0xd60/0x12f8
    [<0000000000a3dc2e>] system_call+0xd6/0x270
    [<ffffffffffffffff>] 0xffffffffffffffff


Turns out kmemleak is right. We now allocate the frontswap map depending on the kernel config
(and no longer on the enablement)

swapfile.c:
[...]
      if (IS_ENABLED(CONFIG_FRONTSWAP))
                frontswap_map = vzalloc(BITS_TO_LONGS(maxpages) * sizeof(long));

but later on this is passed along
--> enable_swap_info(p, prio, swap_map, cluster_info, frontswap_map);

and ignored if frontswap is disabled
--> frontswap_init(p->type, frontswap_map);
static inline void frontswap_init(unsigned type, unsigned long *map)
{
        if (frontswap_enabled())
                __frontswap_init(type, map);
}

Thing is, that frontswap map is never freed.

Christian