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

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Date:   Sat, 16 Apr 2022 00:31:06 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Jesse Barnes <jsbarnes@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>, 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 04:24:14PM -0700, Jesse Barnes wrote:
> On Fri, Apr 15, 2022 at 4:04 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> > 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.

But for those situations, don't you set panic_on_warn anyway?