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Subject[PATCH] hugetlb_cgroup: fix imbalanced css_get and css_put pair 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 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
and only 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.

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 and only hold
one h_cg->css reference, we should do css_get() for each file_region and
release the reference held by caller when they are done.

Fixes: 075a61d07a8e ("hugetlb_cgroup: add accounting for shared mappings")
Signed-off-by: Shijie Luo <luoshijie1@huawei.com>
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: stable@kernel.org
---
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---
include/linux/hugetlb_cgroup.h | 6 +++--
mm/hugetlb.c | 42 ++++++++++++++++++++++++++++++----
mm/hugetlb_cgroup.c | 11 +++++++--
3 files changed, 51 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 9a4b4715faa1..3664cf8b0748 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -280,6 +280,18 @@ 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 (hugetlb_reserve_pages now) will only hold 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 h_cg->css reference. In
+ * order to ensure that one file_region must hold and only
+ * hold 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 +305,14 @@ 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 +336,7 @@ 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 +348,7 @@ 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);
}
}
@@ -659,7 +681,7 @@ 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;
@@ -680,7 +702,7 @@ 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;
@@ -688,13 +710,13 @@ 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;
@@ -5139,6 +5161,10 @@ 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);
@@ -5146,6 +5172,14 @@ bool hugetlb_reserve_pages(struct inode *inode,
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.
+ */
+ css_put(&h_cg->css);
}
}
return true;
diff --git a/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
index f68b51fcda3d..8668ba87cfe4 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 +401,13 @@ 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 and only hold one rg->css
+ * reference.
+ */
+ if (region_del)
+ css_put(rg->css);
}
}

--
2.19.1
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