[PATCH stable 4.14,4.19 1/1] mm: Fix page counter mismatch in shmem_mfill_atomic_pte

[Wupeng Ma <mawupeng1@huawei.com>]

From: Ma Wupeng <mawupeng1@huawei.com>

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 here
					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: 4c27fe4c4c84 ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support")
Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
---
 mm/shmem.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 0788616696dc..0b06724c189e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2339,8 +2339,6 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 	if (ret)
 		goto out_release_uncharge;
 
-	mem_cgroup_commit_charge(page, memcg, false, false);
-
 	_dst_pte = mk_pte(page, dst_vma->vm_page_prot);
 	if (dst_vma->vm_flags & VM_WRITE)
 		_dst_pte = pte_mkwrite(pte_mkdirty(_dst_pte));
@@ -2366,6 +2364,8 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 	if (!pte_none(*dst_pte))
 		goto out_release_uncharge_unlock;
 
+	mem_cgroup_commit_charge(page, memcg, false, false);
+
 	lru_cache_add_anon(page);
 
 	spin_lock_irq(&info->lock);
-- 
2.25.1