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Subject[PATCH RFC] hugetlb_cgroup: fix unbalanced css_put for shared mappings
Date
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 unbalanced css_put().
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.

In order to fix this, we have to make sure that one file_region may hold
and must hold 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.

Fixes: 075a61d07a8e ("hugetlb_cgroup: add accounting for shared mappings")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: stable@kernel.org
---
include/linux/hugetlb_cgroup.h | 6 ++++--
mm/hugetlb.c | 18 ++++++++++++++----
mm/hugetlb_cgroup.c | 10 ++++++++--
3 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/include/linux/hugetlb_cgroup.h b/include/linux/hugetlb_cgroup.h
index 2ad6e92f124a..7610efcd96bd 100644
--- a/include/linux/hugetlb_cgroup.h
+++ b/include/linux/hugetlb_cgroup.h
@@ -138,7 +138,8 @@ 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 +148,8 @@ 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)
{
}

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index a6bad1f686c5..777bc0e45bf3 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -298,6 +298,14 @@ 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)
{
@@ -321,6 +329,7 @@ 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;
@@ -332,6 +341,7 @@ 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);
}
}
@@ -664,7 +674,7 @@ 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;
@@ -685,7 +695,7 @@ 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;
@@ -693,13 +703,13 @@ static long region_del(struct resv_map *resv, long f, long t)

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;
diff --git a/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
index 9182848dda3e..8909e075d441 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 only holds one rg->css reference.
+ */
+ if (region_del)
+ css_put(rg->css);
}
}

--
2.19.1
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