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

[Mina Almasry]

On Mon, Nov 15, 2021 at 10:22 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
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> Subject:   Re: [PATCH v6] hugetlb: Add hugetlb.*.numa_stat file
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> To:        Muchun Song <songmuchun@xxxxxxxxxxxxx>, Shakeel Butt <shakeelb@xxxxxxxxxx>, Mina Almasry <almasrymina@xxxxxxxxxx>
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> 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>
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> Bcc:
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> -=-=-=-=-=-=-=-=-=# Don't remove this line #=-=-=-=-=-=-=-=-=-
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> On 11/14/21 5:43 AM, Muchun Song wrote:
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> > On Sun, Nov 14, 2021 at 3:15 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
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> >> On Sat, Nov 13, 2021 at 6:48 AM Mina Almasry <almasrymina@xxxxxxxxxx> wrote:
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> >>> On Fri, Nov 12, 2021 at 6:45 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
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> >>>> On Sat, Nov 13, 2021 at 7:36 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
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> >> We have following options:
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> >>
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> >> 1) Use atomic type for usage.
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> >> 2) Use "unsigned long" for usage along with WRITE_ONCE/READ_ONCE.
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> >> 3) Use hugetlb_lock for hugetlb_cgroup_read_numa_stat as well.
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> >>
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> >> All options are valid but we would like to avoid (3).
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> >>
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> >> What if we use "unsigned long" type but without READ_ONCE/WRITE_ONCE.
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> >> The potential issues with that are KCSAN will report this as race and
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> >> possible garbage value on archs which do not support atomic writes to
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> >> unsigned long.
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> >
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> > At least I totally agree with you. Thanks for your detailed explanation.
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> >
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>
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> Thanks everyone.  This makes sense.
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>
>
> However, I should note that this same situation (updates to unsigned
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> long variables under lock and reads of the the same variable without
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> lock or READ/WRITE_ONCE) exists in hugetlb sysfs files today.  Not
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> suggesting that this makes it OK to ignore the potential issue.  Just
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> wanted to point this out.
>

Sorry I'm still a bit confused. READ_ONCE/WRITE_ONCE isn't documented
to provide atomicity to the write or read, just prevents the compiler
from re-ordering them. Is there something I'm missing, or is the
suggestion to add READ_ONCE/WRITE_ONCE simply to supress the KCSAN
warnings?

> --
>
> Mike Kravetz
>