hugetlb_cgroup: fix imbalanced css_get and css_put pair for shared mappings

The current implementation of hugetlb_cgroup for shared mappings could
have different behavior.  Consider the following two scenarios:

 1.Assume initial css reference count of hugetlb_cgroup is 1:
  1.1 Call hugetlb_reserve_pages with from = 1, to = 2. So css reference
      count is 2 associated with 1 file_region.
  1.2 Call hugetlb_reserve_pages with from = 2, to = 3. So css reference
      count is 3 associated with 2 file_region.
  1.3 coalesce_file_region will coalesce these two file_regions into
      one. So css reference count is 3 associated with 1 file_region
      now.

 2.Assume initial css reference count of hugetlb_cgroup is 1 again:
  2.1 Call hugetlb_reserve_pages with from = 1, to = 3. So css reference
      count is 2 associated with 1 file_region.

Therefore, we might have one file_region while holding one or more css
reference counts. This inconsistency could lead to imbalanced css_get()
and css_put() pair. If we do css_put one by one (i.g. hole punch case),
scenario 2 would put one more css reference. If we do css_put all
together (i.g. truncate case), scenario 1 will leak one css reference.

The imbalanced css_get() and css_put() pair would result in a non-zero
reference when we try to destroy the hugetlb cgroup. The hugetlb cgroup
directory is removed __but__ associated resource is not freed. This
might result in OOM or can not create a new hugetlb cgroup in a busy
workload ultimately.

In order to fix this, we have to make sure that one file_region must
hold exactly one css reference. So in coalesce_file_region case, we
should release one css reference before coalescence. Also only put css
reference when the entire file_region is removed.

The last thing to note is that the caller of region_add() will only hold
one reference to h_cg->css for the whole contiguous reservation region.
But this area might be scattered when there are already some
file_regions reside in it. As a result, many file_regions may share only
one h_cg->css reference. In order to ensure that one file_region must
hold exactly one css reference, we should do css_get() for each
file_region and release the reference held by caller when they are done.

[linmiaohe@huawei.com: fix imbalanced css_get and css_put pair for shared mappings]
  Link: https://lkml.kernel.org/r/20210316023002.53921-1-linmiaohe@huawei.com

Link: https://lkml.kernel.org/r/20210301120540.37076-1-linmiaohe@huawei.com
Fixes: 075a61d07a8e ("hugetlb_cgroup: add accounting for shared mappings")
Reported-by: kernel test robot <lkp@intel.com> (auto build test ERROR)
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Wanpeng Li <liwp.linux@gmail.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 2 deletions

diff --git a/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
index f68b51fcda3d1..603a131e262dc 100644
--- a/mm/hugetlb_cgroup.c
+++ b/mm/hugetlb_cgroup.c
@@ -391,7 +391,8 @@ void hugetlb_cgroup_uncharge_counter(struct resv_map *resv, unsigned long start,
 
 void hugetlb_cgroup_uncharge_file_region(struct resv_map *resv,
 					 struct file_region *rg,
-					 unsigned long nr_pages)
+					 unsigned long nr_pages,
+					 bool region_del)
 {
 	if (hugetlb_cgroup_disabled() || !resv || !rg || !nr_pages)
 		return;
@@ -400,7 +401,12 @@ void hugetlb_cgroup_uncharge_file_region(struct resv_map *resv,
 	    !resv->reservation_counter) {
 		page_counter_uncharge(rg->reservation_counter,
 				      nr_pages * resv->pages_per_hpage);
-		css_put(rg->css);
+		/*
+		 * Only do css_put(rg->css) when we delete the entire region
+		 * because one file_region must hold exactly one css reference.
+		 */
+		if (region_del)
+			css_put(rg->css);
 	}
 }