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

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Message-Id: <20220415143220.cc37b0b0a368ed2bf2a821f8@linux-foundation.org>
Date:   Fri, 15 Apr 2022 14:32:20 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     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>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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, 15 Apr 2022 14:11:32 -0600 Yu Zhao <yuzhao@...gle.com> wrote:

> >
> > I grabbed
> > https://kojipkgs.fedoraproject.org//packages/kernel/5.18.0/0.rc2.23.fc37/src/kernel-5.18.0-0.rc2.23.fc37.src.rpm
> > and
> 
> Yes, Fedora/RHEL is one concrete example of the model I mentioned
> above (experimental/stable). I added Justin, the Fedora kernel
> maintainer, and he can further clarify.
> 
> If we don't want more VM_BUG_ONs, I'll remove them. But (let me
> reiterate) it seems to me that just defeats the purpose of having
> CONFIG_DEBUG_VM.
> 

Well, I feel your pain.  It was never expected that VM_BUG_ON() would
get subverted in this fashion.

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.

With nice simple semantics, please.  Like "it returns void" and "if you
pass an expression with side-effects then you lose".  And "if you send
a patch which produces warnings when CONFIG_MM_BUG_ON=n then you get to
switch to windows95 for a month".

Let's leave the mglru assertions in place for now and let's think about
creating something more suitable, with a view to switching mglru over
to that at a later time.



But really, none of this addresses the core problem: *_BUG_ON() often
kills the kernel.  So guess what we just did?  We killed the user's
kernel at the exact time when we least wished to do so: when they have
a bug to report to us.  So the thing is self-defeating.

It's much much better to WARN and to attempt to continue.  This makes
it much more likely that we'll get to hear about the kernel flaw.