Re: [PATCH v6] hugetlb: Add hugetlb.*.numa_stat file

[Mike Kravetz]

Subject:   Re: [PATCH v6] hugetlb: Add hugetlb.*.numa_stat file

To:        Muchun Song <songmuchun@xxxxxxxxxxxxx>, Shakeel Butt <shakeelb@xxxxxxxxxx>, Mina Almasry <almasrymina@xxxxxxxxxx>

Cc:        Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Shuah Khan <shuah@xxxxxxxxxx>, Miaohe Lin <linmiaohe@xxxxxxxxxx>, Oscar Salvador <osalvador@xxxxxxx>, Michal Hocko <mhocko@xxxxxxxx>, David Rientjes <rientjes@xxxxxxxxxx>, Jue Wang <juew@xxxxxxxxxx>, Yang Yao <ygyao@xxxxxxxxxx>, Joanna Li <joannali@xxxxxxxxxx>, Cannon Matthews <cannonmatthews@xxxxxxxxxx>, Linux Memory Management List <linux-mm@xxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>

Bcc:       

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On 11/14/21 5:43 AM, Muchun Song wrote:

> On Sun, Nov 14, 2021 at 3:15 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:

>> On Sat, Nov 13, 2021 at 6:48 AM Mina Almasry <almasrymina@xxxxxxxxxx> wrote:

>>> On Fri, Nov 12, 2021 at 6:45 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:

>>>> On Sat, Nov 13, 2021 at 7:36 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:

>> We have following options:

>>

>> 1) Use atomic type for usage.

>> 2) Use "unsigned long" for usage along with WRITE_ONCE/READ_ONCE.

>> 3) Use hugetlb_lock for hugetlb_cgroup_read_numa_stat as well.

>>

>> All options are valid but we would like to avoid (3).

>>

>> What if we use "unsigned long" type but without READ_ONCE/WRITE_ONCE.

>> The potential issues with that are KCSAN will report this as race and

>> possible garbage value on archs which do not support atomic writes to

>> unsigned long.

> 

> At least I totally agree with you. Thanks for your detailed explanation.

> 



Thanks everyone.  This makes sense.



However, I should note that this same situation (updates to unsigned

long variables under lock and reads of the the same variable without

lock or READ/WRITE_ONCE) exists in hugetlb sysfs files today.  Not

suggesting that this makes it OK to ignore the potential issue.  Just

wanted to point this out.

-- 

Mike Kravetz