mm: Fix page counter mismatch in shmem_mfill_atomic_pte

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5JT6V


CVE: NA

--------------------------------

shmem_mfill_atomic_pte() wrongly called mem_cgroup_cancel_charge() in
"success" path, it should mem_cgroup_uncharge() to dec memory counter
instead.
mem_cgroup_cancel_charge() should only be used if this transaction is
unsuccessful and mem_cgroup_uncharge() is used to do this if this
transaction succeed.

This will lead to page->memcg not null and will uncharge one more in
put_page(). The page counter will underflow to maximum value and trigger
oom to kill all process include sshd and leave system unaccessible.

page->memcg is set in the following path:
mem_cgroup_commit_charge
	commit_charge
		page->mem_cgroup = memcg;

extra uncharge will be done in the following path:
put_page
	__put_page
		__put_single_page
			mem_cgroup_uncharge
				if (!page->mem_cgroup) <-- should return
					return
				uncharge_page
				uncharge_batch

To fix this, call mem_cgroup_commit_charge() at the end of this transaction
to make sure this transaction is really finished.

Fixes: 4c27fe4c ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support")
Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>