linux-kernel - Re: [PATCH 01/13] mm: Update ptep_get

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Date:   Thu, 27 Oct 2022 12:43:01 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jann Horn <jannh@...gle.com>, John Hubbard <jhubbard@...dia.com>,
        x86@...nel.org, willy@...radead.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        aarcange@...hat.com, kirill.shutemov@...ux.intel.com,
        jroedel@...e.de, ubizjak@...il.com
Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment

On Thu, Oct 27, 2022 at 12:35 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Oct 27, 2022 at 11:13:55AM -0700, Linus Torvalds wrote:
>
> > But "fullmm" is probably even stronger than "mmap write-lock" in that
> > it should also mean "no other CPU can be actively using this" - either
> > for hardware page table walking, or for GUP.
>
> IIRC fullmm is really: this is the last user and we're taking the whole
> mm down -- IOW exit().

Yes.

But that doesn't mean that it's entirely "just ours" - vmscan can
still see the entries due to rmap, I think. So there can still be some
concurrency concerns, but it's limited.

> Do we worry about CPU errata where things go side-ways if multiple CPUs
> have inconsistent TLB state?

Yeah, we should definitely worry about those, since I think they have
been known to cause machine checks etc, which then crashes the machine
because the machine check architecture is broken garbage.

"User gets the odd memory ordering they asked for" is different from
"user can crash machine because of bad machine check architecture" ;)

That said, I don't think this is a real worry here. Because if I
recall the errata correctly, they are not about "different TLB
contents", but "different cacheability for the same physical page".

Because different TLB contents are normal and even expected, I think.
Things like kmap_local etc already end up doing some lazy TLB
flushing. No?

I think it's only "somebody did an UC access to a cacheline I have"
that ends up being bad.

Note the *WILD HANDWAVING* above - I didn't actually look up the
errata. The above is from my dim memories of the issues we had, and I
might just be wrong.

               Linus