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, 13 insertions, 2 deletions

diff --git a/include/linux/hugetlb_cgroup.h b/include/linux/hugetlb_cgroup.h
index 2ad6e92f124a..0bff345c4bc6 100644
--- a/include/linux/hugetlb_cgroup.h
+++ b/include/linux/hugetlb_cgroup.h
@@ -113,6 +113,11 @@ static inline bool hugetlb_cgroup_disabled(void)
 	return !cgroup_subsys_enabled(hugetlb_cgrp_subsys);
 }
 
+static inline void hugetlb_cgroup_put_rsvd_cgroup(struct hugetlb_cgroup *h_cg)
+{
+	css_put(&h_cg->css);
+}
+
 extern int hugetlb_cgroup_charge_cgroup(int idx, unsigned long nr_pages,
 					struct hugetlb_cgroup **ptr);
 extern int hugetlb_cgroup_charge_cgroup_rsvd(int idx, unsigned long nr_pages,
@@ -138,7 +143,8 @@ extern void hugetlb_cgroup_uncharge_counter(struct resv_map *resv,
 
 extern 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);
 
 extern void hugetlb_cgroup_file_init(void) __init;
 extern void hugetlb_cgroup_migrate(struct page *oldhpage,
@@ -147,7 +153,8 @@ extern void hugetlb_cgroup_migrate(struct page *oldhpage,
 #else
 static inline 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)
 {
 }
 
@@ -185,6 +192,10 @@ static inline bool hugetlb_cgroup_disabled(void)
 	return true;
 }
 
+static inline void hugetlb_cgroup_put_rsvd_cgroup(struct hugetlb_cgroup *h_cg)
+{
+}
+
 static inline int hugetlb_cgroup_charge_cgroup(int idx, unsigned long nr_pages,
 					       struct hugetlb_cgroup **ptr)
 {