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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 19 Oct 2022 10:40:40 -0700
Message-ID: <CAHk-=wj1rc2t5noMtVOgu8XXeTM4KiggEub9PdcexxeQrYPZvA@mail.gmail.com>
Subject: Re: [PATCH v14 08/14] mm: multi-gen LRU: support page table walks
To:     Yu Zhao <yuzhao@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        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>,
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On Tue, Oct 18, 2022 at 10:51 PM Yu Zhao <yuzhao@google.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