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.

[[email protected]: fix imbalanced css_get and css_put pair for shared mappings]
  Link: https://lkml.kernel.org/r/[email protected]

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 075a61d ("hugetlb_cgroup: add accounting for shared mappings")
Reported-by: kernel test robot <[email protected]> (auto build test ERROR)
Signed-off-by: Miaohe Lin <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Wanpeng Li <[email protected]>
Cc: Mina Almasry <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: MiaoheLin

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)
 {

mm/hugetlb.c

@@ -280,6 +280,17 @@ static void record_hugetlb_cgroup_uncharge_info(struct hugetlb_cgroup *h_cg,
 		nrg->reservation_counter =
 			&h_cg->rsvd_hugepage[hstate_index(h)];
 		nrg->css = &h_cg->css;
+		/*
+		 * The caller will hold exactly one h_cg->css reference 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 css
+		 * reference. In order to ensure that one file_region must hold
+		 * exactly one h_cg->css reference, we should do css_get for
+		 * each file_region and leave the reference held by caller
+		 * untouched.
+		 */
+		css_get(&h_cg->css);
 		if (!resv->pages_per_hpage)
 			resv->pages_per_hpage = pages_per_huge_page(h);
 		/* pages_per_hpage should be the same for all entries in
@@ -293,6 +304,14 @@ static void record_hugetlb_cgroup_uncharge_info(struct hugetlb_cgroup *h_cg,
 #endif
 }
 
+static void put_uncharge_info(struct file_region *rg)
+{
+#ifdef CONFIG_CGROUP_HUGETLB
+	if (rg->css)
+		css_put(rg->css);
+#endif
+}
+
 static bool has_same_uncharge_info(struct file_region *rg,
 				   struct file_region *org)
 {
@@ -316,6 +335,7 @@ static void coalesce_file_region(struct resv_map *resv, struct file_region *rg)
 		prg->to = rg->to;
 
 		list_del(&rg->link);
+		put_uncharge_info(rg);
 		kfree(rg);
 
 		rg = prg;
@@ -327,6 +347,7 @@ static void coalesce_file_region(struct resv_map *resv, struct file_region *rg)
 		nrg->from = rg->from;
 
 		list_del(&rg->link);
+		put_uncharge_info(rg);
 		kfree(rg);
 	}
 }
@@ -662,7 +683,7 @@ static long region_del(struct resv_map *resv, long f, long t)
 
 			del += t - f;
 			hugetlb_cgroup_uncharge_file_region(
-				resv, rg, t - f);
+				resv, rg, t - f, false);
 
 			/* New entry for end of split region */
 			nrg->from = t;
@@ -683,21 +704,21 @@ static long region_del(struct resv_map *resv, long f, long t)
 		if (f <= rg->from && t >= rg->to) { /* Remove entire region */
 			del += rg->to - rg->from;
 			hugetlb_cgroup_uncharge_file_region(resv, rg,
-							    rg->to - rg->from);
+							    rg->to - rg->from, true);
 			list_del(&rg->link);
 			kfree(rg);
 			continue;
 		}
 
 		if (f <= rg->from) {	/* Trim beginning of region */
 			hugetlb_cgroup_uncharge_file_region(resv, rg,
-							    t - rg->from);
+							    t - rg->from, false);
 
 			del += t - rg->from;
 			rg->from = t;
 		} else {		/* Trim end of region */
 			hugetlb_cgroup_uncharge_file_region(resv, rg,
-							    rg->to - f);
+							    rg->to - f, false);
 
 			del += rg->to - f;
 			rg->to = f;
@@ -5187,13 +5208,25 @@ bool hugetlb_reserve_pages(struct inode *inode,
 			 */
 			long rsv_adjust;
 
+			/*
+			 * hugetlb_cgroup_uncharge_cgroup_rsvd() will put the
+			 * reference to h_cg->css. See comment below for detail.
+			 */
 			hugetlb_cgroup_uncharge_cgroup_rsvd(
 				hstate_index(h),
 				(chg - add) * pages_per_huge_page(h), h_cg);
 
 			rsv_adjust = hugepage_subpool_put_pages(spool,
 								chg - add);
 			hugetlb_acct_memory(h, -rsv_adjust);
+		} else if (h_cg) {
+			/*
+			 * The file_regions will hold their own reference to
+			 * h_cg->css. So we should release the reference held
+			 * via hugetlb_cgroup_charge_cgroup_rsvd() when we are
+			 * done.
+			 */
+			hugetlb_cgroup_put_rsvd_cgroup(h_cg);
 		}
 	}
 	return true;

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);
 	}
 }