mm: memcontrol: rewrite uncharge API fix 2

mm: memcontrol: rewrite uncharge API fix 2

It's not entirely clear whether do_swap_account or PCG_MEMSW is the
authoritative answer to whether a page is swap-accounted or not.  This
currently leads to the following memsw counter underflow when swap
accounting is disabled:

[    2.753355] WARNING: CPU: 0 PID: 1 at kernel/res_counter.c:28 res_counter_uncharge_locked+0x48/0x74()
[    2.753355] CPU: 0 PID: 1 Comm: init Not tainted 3.16.0-rc1-00238-gddc5bfe #1
[    2.753355] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[    2.753355]  0000000000000000 ffff880012073c50 ffffffff81a23b9d ffff880012073c88
[    2.753355]  ffffffff810bc765 ffffffff8111fac8 0000000000001000 ffff88001200fa50
[    2.753355]  0000000000000001 ffff88001200fa01 ffff880012073c98 ffffffff810bc84b
[    2.753355] Call Trace:
[    2.753355]  [<ffffffff81a23b9d>] dump_stack+0x19/0x1b
[    2.753355]  [<ffffffff810bc765>] warn_slowpath_common+0x73/0x8c
[    2.753355]  [<ffffffff8111fac8>] ? res_counter_uncharge_locked+0x48/0x74
[    2.753355]  [<ffffffff810bc84b>] warn_slowpath_null+0x1a/0x1c
[    2.753355]  [<ffffffff8111fac8>] res_counter_uncharge_locked+0x48/0x74
[    2.753355]  [<ffffffff8111fd02>] res_counter_uncharge_until+0x4e/0xa9
[    2.753355]  [<ffffffff8111fd70>] res_counter_uncharge+0x13/0x15
[    2.753355]  [<ffffffff8119499c>] mem_cgroup_uncharge_end+0x73/0x8d
[    2.753355]  [<ffffffff8115735e>] release_pages+0x1f2/0x20d
[    2.753355]  [<ffffffff8116cc3a>] tlb_flush_mmu_free+0x28/0x43
[    2.753355]  [<ffffffff8116d5e5>] tlb_flush_mmu+0x20/0x23
[    2.753355]  [<ffffffff8116d5fc>] tlb_finish_mmu+0x14/0x39
[    2.753355]  [<ffffffff811730c1>] unmap_region+0xcd/0xdf
[    2.753355]  [<ffffffff81172b0e>] ? vma_gap_callbacks_propagate+0x18/0x33
[    2.753355]  [<ffffffff81174bf1>] do_munmap+0x252/0x2e0
[    2.753355]  [<ffffffff81174cc3>] vm_munmap+0x44/0x5c
[    2.753355]  [<ffffffff81174cfe>] SyS_munmap+0x23/0x29
[    2.753355]  [<ffffffff81a31567>] system_call_fastpath+0x16/0x1b
[    2.753355] ---[ end trace cfeb07101f6fbdfb ]---

Don't set PCG_MEMSW when swap accounting is disabled, so that uncharging
only has to look at this per-page flag.

mem_cgroup_swapout() could also fully rely on this flag, but as it can
bail out before even looking up the page_cgroup, check do_swap_account as
a performance optimization and only sanity test for PCG_MEMSW.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Michal Hocko <mhocko@suse.cz>
Tested-by: Jet Chen <jet.chen@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>