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

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Yu Zhao <yuzhao@google.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"Justin Forbes" <jforbes@fedoraproject.org>,
	"Stephen Rothwell" <sfr@rothwell.id.au>,
	Linux-MM <linux-mm@kvack.org>, "Andi Kleen" <ak@linux.intel.com>,
	"Aneesh Kumar" <aneesh.kumar@linux.ibm.com>,
	"Barry Song" <21cnbao@gmail.com>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Hillf Danton" <hdanton@sina.com>, "Jens Axboe" <axboe@kernel.dk>,
	"Jesse Barnes" <jsbarnes@google.com>,
	"Johannes Weiner" <hannes@cmpxchg.org>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Mel Gorman" <mgorman@suse.de>,
	"Michael Larabel" <Michael@michaellarabel.com>,
	"Michal Hocko" <mhocko@kernel.org>,
	"Mike Rapoport" <rppt@kernel.org>,
	"Rik van Riel" <riel@surriel.com>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	"Will Deacon" <will@kernel.org>,
	"Ying Huang" <ying.huang@intel.com>,
	"Linux ARM" <linux-arm-kernel@lists.infradead.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	"Kernel Page Reclaim v2" <page-reclaim@google.com>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	"Brian Geffon" <bgeffon@google.com>,
	"Jan Alexander Steffens" <heftig@archlinux.org>,
	"Oleksandr Natalenko" <oleksandr@natalenko.name>,
	"Steven Barrett" <steven@liquorix.net>,
	"Suleiman Souhlal" <suleiman@google.com>,
	"Daniel Byrne" <djbyrne@mtu.edu>,
	"Donald Carr" <d@chaos-reins.com>,
	"Holger Hoffstätte" <holger@applied-asynchrony.com>,
	"Konstantin Kharlamov" <Hi-Angel@yandex.ru>,
	"Shuang Zhai" <szhai2@cs.rochester.edu>,
	"Sofia Trinh" <sofia.trinh@edi.works>,
	"Vaibhav Jain" <vaibhav@linux.ibm.com>
Subject: Re: [PATCH v10 08/14] mm: multi-gen LRU: support page table walks
Date: Fri, 15 Apr 2022 16:03:28 -0700	[thread overview]
Message-ID: <CAHk-=whneDk3Jde3J+O-fD32VjaK+fDf9+P6jgDtr2qyo0iu2w@mail.gmail.com> (raw)
In-Reply-To: <CAOUHufZ4KrjFTYH8wtwMGd9AriZfZtO4GhbiK1SuNbY31VTT9w@mail.gmail.com>

On Fri, Apr 15, 2022 at 3:58 PM Yu Zhao <yuzhao@google.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.

                  Linus