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>

diff --git a/include/linux/hugetlb_cgroup.h b/include/linux/hugetlb_cgroup.h
index 2ad6e92..0bff345 100644 (file)
--- 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)
 {

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 5b1ab1f..a86a58e 100644 (file)
--- a/mm/hugetlb.c
+++ b/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 @@ retry:
 
                        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;
@@ -683,7 +704,7 @@ retry:
                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;
@@ -691,13 +712,13 @@ retry:
 
                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;
@@ -5187,6 +5208,10 @@ 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);
@@ -5194,6 +5219,14 @@ 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.
+                        */
+                       hugetlb_cgroup_put_rsvd_cgroup(h_cg);
                }
        }
        return true;

diff --git a/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
index f68b51f..603a131 100644 (file)
--- 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);
        }
 }