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

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Message-ID: <CAJmaN=mJKAhRjDHbsLrqyK_uvuysx596jEcTwV2fwx4UOe4oow@mail.gmail.com>
Date:   Fri, 15 Apr 2022 16:24:14 -0700
From:   Jesse Barnes <jsbarnes@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Yu Zhao <yuzhao@...gle.com>,
        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>,
        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: [page-reclaim] Re: [PATCH v10 08/14] mm: multi-gen LRU: support
 page table walks

On Fri, Apr 15, 2022 at 4:04 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Fri, Apr 15, 2022 at 3:58 PM Yu Zhao <yuzhao@...gle.com> wrote:
> >
> > 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 don't understand your argument.
>
> First you say that VM_BUG_ON() is only for VM developers.
>
> Then you say "some testers (ordinary users) ignore WARN_ON_ONCEs until
> the kernel crashes".
>
> So which is it?
>
> VM developers, or ordinary users?
>
> Honestly, if a VM developer is ignoring a WARN_ON_ONCE() from the VM
> subsystem, I don't even know what to say.
>
> And for ordinary users, a WARN_ON_ONCE() is about a million times
> better, becasue:
>
>  - the machine will hopefully continue working, so they can report the warning
>
>  - even when they don't notice them, distros tend to have automated
> reporting infrastructure
>
> That's why I absolutely *DETEST* those stupid BUG_ON() cases - they
> will often kill the machine with nasty locks held, resulting in a
> completely undebuggable thing that never gets reported.
>
> Yes, you can be careful and only put BUG_ON() in places where recovery
> is possible. But even then, they have no actual _advantages_ over just
> a WARN_ON_ONCE.

Generally agreed, and not to belabor this relatively small issue, but in some
environments like cloud or managed client deployments, a crash can actually
be preferable so we can get a dump, reboot the machine, and get things going
again for the application or user, then debug offline.  So having the
flexibility to
do that in those situations is helpful.  And there, a full crash dump is better
than just a log report with the WARN info, since debugging may be easier with
all the kernel memory.

Jesse