mm: mkfs.ext4 invoked oom-killer on i386 - pagecache_get_page

Fix crash with cgroup_disable=memory:

> > > > + mkfs -t ext4 /dev/disk/by-id/ata-TOSHIBA_MG04ACA100N_Y8NRK0BPF6XF
> > > > mke2fs 1.43.8 (1-Jan-2018)
> > > > Creating filesystem with 244190646 4k blocks and 61054976 inodes
> > > > Filesystem UUID: 3bb1a285-2cb4-44b4-b6e8-62548f3ac620
> > > > Superblock backups stored on blocks:
> > > > 32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
> > > > 4096000, 7962624, 11239424, 20480000, 23887872, 71663616, 78675968,
> > > > 102400000, 214990848
> > > > Allocating group tables:    0/7453                           done
> > > > Writing inode tables:    0/7453                           done
> > > > Creating journal (262144 blocks): [   35.502102] BUG: kernel NULL
> > > > pointer dereference, address: 000000c8
> > > > [   35.508372] #PF: supervisor read access in kernel mode
> > > > [   35.513506] #PF: error_code(0x0000) - not-present page
> > > > [   35.518638] *pde = 00000000
> > > > [   35.521514] Oops: 0000 [#1] SMP
> > > > [   35.524652] CPU: 0 PID: 145 Comm: kswapd0 Not tainted
> > > > 5.7.0-rc6-next-20200519+ #1
> > > > [   35.532121] Hardware name: Supermicro SYS-5019S-ML/X11SSH-F, BIOS
> > > > 2.2 05/23/2018
> > > > [   35.539507] EIP: mem_cgroup_get_nr_swap_pages+0x28/0x60

Swap accounting used to be implied-disabled when the cgroup controller was
disabled.  Restore that for the new cgroup_memory_noswap, so that we bail
out of this function instead of dereferencing a NULL memcg.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Johannes Weiner <[email protected]>
Reported-by: Naresh Kamboju <[email protected]>
Debugged-by: Hugh Dickins <[email protected]>
Debugged-by: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Stephen Rothwell <[email protected]>

mm/memcontrol.c

@@ -7152,7 +7152,11 @@ static struct cftype memsw_files[] = {
 
 static int __init mem_cgroup_swap_init(void)
 {
-	if (mem_cgroup_disabled() || cgroup_memory_noswap)
+	/* No memory control -> no swap control */
+	if (mem_cgroup_disabled())
+		cgroup_memory_noswap = true;
+
+	if (cgroup_memory_noswap)
 		return 0;
 
 	WARN_ON(cgroup_add_dfl_cftypes(&memory_cgrp_subsys, swap_files));