Subject: Re: [PATCH v6] hugetlb: Add hugetlb.*.numa_stat file To: Muchun Song <songmuchun@xxxxxxxxxxxxx>, Shakeel Butt <shakeelb@xxxxxxxxxx>, Mina Almasry <almasrymina@xxxxxxxxxx> 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> 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@xxxxxxxxxx> wrote: >> On Sat, Nov 13, 2021 at 6:48 AM Mina Almasry <almasrymina@xxxxxxxxxx> wrote: >>> On Fri, Nov 12, 2021 at 6:45 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote: >>>> On Sat, Nov 13, 2021 at 7:36 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> 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
