linux-kernel - Re: [PATCH v10 08/14] mm: multi-gen LRU: support page table walks

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Date:   Fri, 15 Apr 2022 16:57:37 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Justin Forbes <jforbes@...oraproject.org>,
        Stephen Rothwell <sfr@...hwell.id.au>,
        Linux-MM <linux-mm@...ck.org>, Andi Kleen <ak@...ux.intel.com>,
        Aneesh Kumar <aneesh.kumar@...ux.ibm.com>,
        Barry Song <21cnbao@...il.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 <linux-kernel@...r.kernel.org>,
        Kernel Page Reclaim v2 <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 v10 08/14] mm: multi-gen LRU: support page table walks

On Fri, Apr 15, 2022 at 3:43 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Fri, Apr 15, 2022 at 2:32 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> >
> > We could create a new MM-developer-only assertion.  Might even call it
> > MM_BUG_ON().  With compile-time enablement but perhaps not a runtime
> > switch.
>
> .. or VM_BUG_ON() could just become a WARN_ON_ONCE().
>
> Which it should be anyway - since the code has to be written to
> continue after that BUG_ON() anyway.
>
> There is absolutely _zero_ advantage to killing the machine. If you
> want to be notified about "this must not happen", then WARN_ON_ONCE()
> is the right thing to use.
>
> BUG_ON() is basically always the wrong thing to do.

Not trying to start a meta discussion, just my two cents:

This is a typical trolley problem: for the greater good, do we want to
inflict more pain on a small group of users running experimental
kernels so that they'd come back and yell at us quicker and louder?
BUG_ONs are harmful but problems that trigger them would be
presummingly less penetrating to the user base; on the other hand,
from my experience working with some testers (ordinary users), they
ignore WARN_ON_ONCEs until the kernel crashes.

I'll let Justin chime in on Fedora's take on CONFIG_DEBUG_VM. I bet
it's intended to crash the kernel.