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

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Date:   Mon, 24 Oct 2022 15:20:55 -0300
From:   Gareth Poole <girpoole@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>
Cc:     Yu Zhao <yuzhao@...gle.com>,
        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,
        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

As someone who still regularly uses hardware from this era, and often 
runs Linux on it, this would definitely be a blow to which machines I 
can actively use. Linux support is a big part of how I use these 
machines, since DOS and Windows 95 really can’t keep up with modern 
networking standards.

I would be very disappointed, and impacted, if Linux dropped 486 support.

On 10/20/22 23:10, Linus Torvalds wrote:
> On Thu, Oct 20, 2022 at 11:55 AM Peter Zijlstra <peterz@...radead.org> wrote:
>> On Thu, Oct 20, 2022 at 10:35:28AM -0700, Linus Torvalds wrote:
>>> That said, I reacted to that cmpxchg loop:
>>>
>>>          } while (cmpxchg64(&pmdp->pmd, old.pmd, 0ULL) != old.pmd);
>>>
>>> is this series just so old (but rebased) that it doesn't use "try_cmpxchg64()"?
>> Yep -- it's *that* old :-/ You're not in fact the first to point that
>> out.
>>
>> I'll make time tomorrow to fix it up and respin and send out. This is as
>> good a time as any to get rid of carrying these patches myself.
> Hmm. Thinking some more about it, even if you do it as a
> try_cmpxchg64() loop, it ends up being duplicated twice.
>
> Maybe we should just bite the bullet, and say that we only support
> x86-32 with 'cmpxchg8b' (ie Pentium and later).
>
> Get rid of all the "emulate 64-bit atomics with cli/sti, knowing that
> nobody has SMP on those CPU's anyway", and implement a generic x86-32
> xchg() setup using that try_cmpxchg64 loop.
>
> I think most (all?) distros already enable X86_PAE anyway, which makes
> that X86_CMPXCHG64 be part of the base requirement.
>
> Not that I'm convinced most distros even do 32-bit development anyway
> these days.
>
> (Of course, if we require X86_CMPXCHG64, we'll also hit some of the
> odd clone CPU's that actually *do* support the instruction, but do not
> report it in cpuid due to an odd old Windows NT bug. IOW, things like
> the Cyrix and Transmeta CPU's did support the instruction, but had the
> CX8 bit clear because otherwise NT wouldn't boot. We may or may not
> get those cases right, but I doubt anybody really has any of those old
> CPUs).
>
> We got rid of i386 support back in 2012. Maybe it's time to get rid of
> i486 support in 2022?
>
> That way we could finally get rid of CONFIG_MATH_EMULATION too.
>
>                 Linus
>