linux-kernel - Re: [PATCH v6] hugetlb: Add hugetlb.*.numa

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Date:   Mon, 15 Nov 2021 10:55:41 -0800
From:   Mina Almasry <almasrymina@...gle.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Jue Wang <juew@...gle.com>, Yang Yao <ygyao@...gle.com>,
        Joanna Li <joannali@...gle.com>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6] hugetlb: Add hugetlb.*.numa_stat file

On Mon, Nov 15, 2021 at 10:22 AM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>
> Subject:   Re: [PATCH v6] hugetlb: Add hugetlb.*.numa_stat file
>
> To:        Muchun Song <songmuchun@...edance.com>, Shakeel Butt <shakeelb@...gle.com>, Mina Almasry <almasrymina@...gle.com>
>
> Cc:        Andrew Morton <akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>, Miaohe Lin <linmiaohe@...wei.com>, Oscar Salvador <osalvador@...e.de>, Michal Hocko <mhocko@...e.com>, David Rientjes <rientjes@...gle.com>, Jue Wang <juew@...gle.com>, Yang Yao <ygyao@...gle.com>, Joanna Li <joannali@...gle.com>, Cannon Matthews <cannonmatthews@...gle.com>, Linux Memory Management List <linux-mm@...ck.org>, LKML <linux-kernel@...r.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@...gle.com> wrote:
>
> >> On Sat, Nov 13, 2021 at 6:48 AM Mina Almasry <almasrymina@...gle.com> wrote:
>
> >>> On Fri, Nov 12, 2021 at 6:45 PM Muchun Song <songmuchun@...edance.com> wrote:
>
> >>>> On Sat, Nov 13, 2021 at 7:36 AM Mike Kravetz <mike.kravetz@...cle.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.
>

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
>