linux-kernel - Re: [PATCH v14 08/14] mm: multi-gen LRU: support page table walks

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Date:   Wed, 19 Oct 2022 10:40:40 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Aneesh Kumar <aneesh.kumar@...ux.ibm.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>, Jens Axboe <axboe@...nel.dk>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Michael Larabel <Michael@...haellarabel.com>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...nel.org>, Tejun Heo <tj@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        page-reclaim@...gle.com, Brian Geffon <bgeffon@...gle.com>,
        Jan Alexander Steffens <heftig@...hlinux.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Steven Barrett <steven@...uorix.net>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Daniel Byrne <djbyrne@....edu>,
        Donald Carr <d@...os-reins.com>,
        Holger Hoffstätte <holger@...lied-asynchrony.com>,
        Konstantin Kharlamov <Hi-Angel@...dex.ru>,
        Shuang Zhai <szhai2@...rochester.edu>,
        Sofia Trinh <sofia.trinh@....works>,
        Vaibhav Jain <vaibhav@...ux.ibm.com>
Subject: Re: [PATCH v14 08/14] mm: multi-gen LRU: support page table walks

On Tue, Oct 18, 2022 at 10:51 PM Yu Zhao <yuzhao@...gle.com> wrote:
>
> pmd_read_atomic() should have a built-in READ_ONCE() in the first
> place.

I really think that is the right thing to do. Let's just move the
barrier in *there* instead.

It really should use 'READ_ONCE()', but it sadly cannot do that
portably, because 'pmd_t' may be a multi-word structure.

Of course, the x86-32 code does this all *almost* right, and
implements its own version of pmd_read_atomic(), but then sadly does
it _without_ actually using READ_ONCE() there.

So even when we could do it right, we don't.

But the x86-32 implementation of pmd_read_atomic() would be trivial to
fix to just use READ_ATOMIC, and the generic implementation should
just have a "barrier()" in it so that we wouldn't need crazy barriers
in the users.

Because as you say, the function is already called "read_atomic", and
it should damn well *act* that way then.

Hmm?

                 Linus