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SubjectRe: [PATCH] mm: Convert mem_cgroup_id::ref to refcount_t type
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Hi, Andrea,

On 16.10.2018 15:49, Andrea Parri wrote:
> Hi Kirill,
>
> On Tue, Oct 09, 2018 at 08:46:56PM +0300, Kirill Tkhai wrote:
>> This will allow to use generic refcount_t interfaces
>> to check counters overflow instead of currently existing
>> VM_BUG_ON(). The only difference after the patch is
>> VM_BUG_ON() may cause BUG(), while refcount_t fires
>> with WARN().
>
> refcount_{sub_and_test,inc_not_zero}() are documented to provide
> "slightly" more relaxed ordering than their atomic_* counterpart,
> c.f.,
>
> Documentation/core-api/refcount-vs-atomic.rst
> lib/refcount.c (inline comments)
>
> IIUC, this semantic change won't cause problems here (but please
> double-check? ;D ).

I just don't see a place, where we may think about using a modification
of struct mem_cgroup::id::ref as a memory barrier to order something,
and all this looks safe for me.

Kirill

>> But this seems not to be significant here,
>> since such the problems are usually caught by syzbot
>> with panic-on-warn enabled.
>>
>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>> ---
>> include/linux/memcontrol.h | 2 +-
>> mm/memcontrol.c | 10 ++++------
>> 2 files changed, 5 insertions(+), 7 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index 4399cc3f00e4..7ab2120155a4 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -78,7 +78,7 @@ struct mem_cgroup_reclaim_cookie {
>>
>> struct mem_cgroup_id {
>> int id;
>> - atomic_t ref;
>> + refcount_t ref;
>> };
>>
>> /*
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 7bebe2ddec05..aa728d5b3d72 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -4299,14 +4299,12 @@ static void mem_cgroup_id_remove(struct mem_cgroup *memcg)
>>
>> static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n)
>> {
>> - VM_BUG_ON(atomic_read(&memcg->id.ref) <= 0);
>> - atomic_add(n, &memcg->id.ref);
>> + refcount_add(n, &memcg->id.ref);
>> }
>>
>> static void mem_cgroup_id_put_many(struct mem_cgroup *memcg, unsigned int n)
>> {
>> - VM_BUG_ON(atomic_read(&memcg->id.ref) < n);
>> - if (atomic_sub_and_test(n, &memcg->id.ref)) {
>> + if (refcount_sub_and_test(n, &memcg->id.ref)) {
>> mem_cgroup_id_remove(memcg);
>>
>> /* Memcg ID pins CSS */
>> @@ -4523,7 +4521,7 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
>> }
>>
>> /* Online state pins memcg ID, memcg ID pins CSS */
>> - atomic_set(&memcg->id.ref, 1);
>> + refcount_set(&memcg->id.ref, 1);
>> css_get(css);
>> return 0;
>> }
>> @@ -6357,7 +6355,7 @@ subsys_initcall(mem_cgroup_init);
>> #ifdef CONFIG_MEMCG_SWAP
>> static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
>> {
>> - while (!atomic_inc_not_zero(&memcg->id.ref)) {
>> + while (!refcount_inc_not_zero(&memcg->id.ref)) {
>> /*
>> * The root cgroup cannot be destroyed, so it's refcount must
>> * always be >= 1.
>>

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