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From:   Yu Zhao <yuzhao@google.com>
Date:   Tue, 18 Oct 2022 23:51:00 -0600
Message-ID: <CAOUHufa9+FTO3Pv-5jC-e3S5goPsUGu-5KcPVHa4bWb0X+d2ug@mail.gmail.com>
Subject: Re: [PATCH v14 08/14] mm: multi-gen LRU: support page table walks
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        Aneesh Kumar <aneesh.kumar@linux.ibm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Hillf Danton <hdanton@sina.com>, Jens Axboe <axboe@kernel.dk>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Michael Larabel <Michael@michaellarabel.com>,
        Michal Hocko <mhocko@kernel.org>,
        Mike Rapoport <rppt@kernel.org>, Tejun Heo <tj@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Will Deacon <will@kernel.org>,
        linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
        page-reclaim@google.com, Brian Geffon <bgeffon@google.com>,
        Jan Alexander Steffens <heftig@archlinux.org>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        Steven Barrett <steven@liquorix.net>,
        Suleiman Souhlal <suleiman@google.com>,
        Daniel Byrne <djbyrne@mtu.edu>,
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        =?UTF-8?Q?Holger_Hoffst=C3=A4tte?= <holger@applied-asynchrony.com>,
        Konstantin Kharlamov <Hi-Angel@yandex.ru>,
        Shuang Zhai <szhai2@cs.rochester.edu>,
        Sofia Trinh <sofia.trinh@edi.works>,
        Vaibhav Jain <vaibhav@linux.ibm.com>
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On Thu, Oct 13, 2022 at 9:04 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Aug 15, 2022 at 01:13:27AM -0600, Yu Zhao wrote:
> > +     for (i = pmd_index(start), addr = start; addr != end; i++, addr = next) {
> > +             pmd_t val = pmd_read_atomic(pmd + i);
> > +
> > +             /* for pmd_read_atomic() */
> > +             barrier();
>
> Please clarify the above. This is an entirely inadequate ordering
> comment.

If it's acceptable, I'll copy what we have in
pmd_none_or_clear_bad_unless_trans_huge():

        pmd_t pmdval = pmd_read_atomic(pmd);

       /* See pmd_none_or_trans_huge_or_clear_bad for info on barrier */
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
        barrier();
#endif

        if (pmd_none(pmdval))
                return 1;

pmd_read_atomic() should have a built-in READ_ONCE() in the first
place. If we have to use pmd_read_atomic(), it means we are not under
PMD PTL. So we can also race with pte_alloc(), regardless of THP
split. In this case, compiler reordering probably won't cause any real
damage, but technically not having barrier() is still a bug and will
trigger KCSAN warnings, I think.