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

[Shakeel Butt <shakeelb@google.com>]

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On Mon, Nov 15, 2021 at 10:55 AM Mina Almasry <almasrymina@google.com> wrote:
>
> On Mon, Nov 15, 2021 at 10:22 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
> >
> > 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.
> >
>
> 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?
>

+Paul & Marco

Let's ask the experts.

We have a "unsigned long usage" variable that is updated within a lock
(hugetlb_lock) but is read without the lock.

Q1) I think KCSAN will complain about it and READ_ONCE() in the
unlocked read path should be good enough to silent KCSAN. So, the
question is should we still use WRITE_ONCE() as well for usage within
hugetlb_lock?

Q2) Second question is more about 64 bit archs breaking a 64 bit write
into two 32 bit writes. Is this a real issue? If yes, then the
combination of READ_ONCE()/WRITE_ONCE() are good enough for the given
use-case?

thanks,
Shakeel