linux-kernel - Re: [PATCH v8 06/14] mm: multi-gen LRU: minimal implementation

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Date:   Tue, 8 Mar 2022 16:02:29 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     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>,
        Jesse Barnes <jsbarnes@...gle.com>,
        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>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        Ying Huang <ying.huang@...el.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, page-reclaim@...gle.com,
        "the arch/x86 maintainers" <x86@...nel.org>,
        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 v8 06/14] mm: multi-gen LRU: minimal implementation

On Tue, Mar 8, 2022 at 3:48 PM Yu Zhao <yuzhao@...gle.com> wrote:
> +
> +config TIERS_PER_GEN
> +       int "Number of tiers per generation"
> +       depends on LRU_GEN
> +       range 2 4
> +       default 4
> +       help
> +         Do not decrease this value unless you run out of spare bits in page
> +         flags, i.e., you see the "Not enough bits in page flags" build error.
> +
> +         This option uses N-2 bits in page flags.

Exact same issue as with the previous patch. Don't ask things like
this. Most *definitely* don't ask things like this if they can cause
build errors.

Just set the tiers to 4, and make sure that the number of generations
is small enough that the "Not enough bits in page flags" build error
just cannot happen.

This kind of "ask people questions they cannot sanely answer" is not acceptable.

And build errors that depend on configuration also aren't acceptable.

End result: DO NOT DO THIS.

The whole "ask user a question that you can't answer yourself" is an
actively wrong cop-out.

If you can't answer it, then the user sure as hell can't either, and
the question is pure garbage and only results in more problems and
less coherent testing.

                      Linus