Subject: Re: [PATCH v6] hugetlb: Add hugetlb.*.numa_stat file
To:     Muchun Song <songmuchun@bytedance.com>,
        Shakeel Butt <shakeelb@google.com>,
        Mina Almasry <almasrymina@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Shuah Khan <shuah@kernel.org>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Oscar Salvador <osalvador@suse.de>,
        Michal Hocko <mhocko@suse.com>,
        David Rientjes <rientjes@google.com>,
        Jue Wang <juew@google.com>, Yang Yao <ygyao@google.com>,
        Joanna Li <joannali@google.com>,
        Cannon Matthews <cannonmatthews@google.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
References: <20211111015037.4092956-1-almasrymina@google.com>
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From:   Mike Kravetz <mike.kravetz@oracle.com>
Message-ID: <6887a91a-9ec8-e06e-4507-b2dff701a147@oracle.com>
Date:   Mon, 15 Nov 2021 10:22:03 -0800
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Subject:   Re: [PATCH v6] hugetlb: Add hugetlb.*.numa_stat file

To:        Muchun Song <songmuchun@bytedance.com>, Shakeel Butt <shakeelb@google.com>, Mina Almasry <almasrymina@google.com>

Cc:        Andrew Morton <akpm@linux-foundation.org>, Shuah Khan <shuah@kernel.org>, Miaohe Lin <linmiaohe@huawei.com>, Oscar Salvador <osalvador@suse.de>, Michal Hocko <mhocko@suse.com>, David Rientjes <rientjes@google.com>, Jue Wang <juew@google.com>, Yang Yao <ygyao@google.com>, Joanna Li <joannali@google.com>, Cannon Matthews <cannonmatthews@google.com>, Linux Memory Management List <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>

Bcc:       

-=-=-=-=-=-=-=-=-=# Don't remove this line #=-=-=-=-=-=-=-=-=-

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

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

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

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

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