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

[Muchun Song <songmuchun@bytedance.com>]

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:
> > > >
> > > > Subject:   Re: [PATCH v6] hugetlb: Add hugetlb.*.numa_stat file
> > > >
> > > > To:        Muchun Song <songmuchun@bytedance.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>, Shakeel Butt <shakeelb@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 11/10/21 6:36 PM, Muchun Song wrote:
> > > >
> > > > > On Thu, Nov 11, 2021 at 9:50 AM Mina Almasry <almasrymina@google.com> wrote:
> > > >
> > > > >>
> > > >
> > > > >> +struct hugetlb_cgroup_per_node {
> > > >
> > > > >> +       /* hugetlb usage in pages over all hstates. */
> > > >
> > > > >> +       atomic_long_t usage[HUGE_MAX_HSTATE];
> > > >
> > > > >
> > > >
> > > > > Why do you use atomic? IIUC, 'usage' is always
> > > >
> > > > > increased/decreased under hugetlb_lock except
> > > >
> > > > > hugetlb_cgroup_read_numa_stat() which is always
> > > >
> > > > > reading it. So I think WRITE_ONCE/READ_ONCE
> > > >
> > > > > is enough.
> > > >
> > > >
> > > >
> > > > Thanks for continuing to work this, I was traveling and unable to
> > > >
> > > > comment.
> > >
> > > Have a good time.
> > >
> > > >
> > > >
> > > >
> > > > Unless I am missing something, I do not see a reason for WRITE_ONCE/READ_ONCE
> > >
> > > Because __hugetlb_cgroup_commit_charge and
> > > hugetlb_cgroup_read_numa_stat can run parallely,
> > > which meets the definition of data race. I believe
> > > KCSAN could report this race. I'm not strongly
> > > suggest using WRITE/READ_ONCE() here. But
> > > in theory it should be like this. Right?
> > >
> >
> > My understanding is that the (only) potential problem here is
> > read_numa_stat() reading an intermediate garbage value while
> > commit_charge() is happening concurrently. This will only happen on
> > archs where the writes to an unsigned long aren't atomic. On archs
> > where writes to an unsigned long are atomic, there is no race, because
> > read_numa_stat() will only ever read the value before the concurrent
> > write or after the concurrent write, both of which are valid. To cater
> > to archs where the writes to unsigned long aren't atomic, we need to
> > use an atomic data type.
> >
> > I'm not too familiar but my understanding from reading the
> > documentation is that WRITE_ONCE/READ_ONCE don't contribute anything
> > meaningful here:
> >
> > /*
> > * Prevent the compiler from merging or refetching reads or writes. The
> > * compiler is also forbidden from reordering successive instances of
> > * READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
> > * particular ordering. One way to make the compiler aware of ordering is to
> > * put the two invocations of READ_ONCE or WRITE_ONCE in different C
> > * statements.
> > ...
> >
> > I can't see a reason why we care about the compiler merging or
> > refetching reads or writes here. As far as I can tell the problem is
> > atomicy of the write.
> >
>
> 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.

>
> Shakeel