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

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Date:   Fri, 15 Apr 2022 12:15:21 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     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 00:25:45 -0600 Yu Zhao <yuzhao@...gle.com> wrote:

> On Thu, Apr 14, 2022 at 7:57 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> >
> > On Thu, 14 Apr 2022 19:14:54 -0600 Yu Zhao <yuzhao@...gle.com> wrote:
> >
> > > On Mon, Apr 11, 2022 at 8:16 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> > > >
> > > > On Wed,  6 Apr 2022 21:15:20 -0600 Yu Zhao <yuzhao@...gle.com> wrote:
> > > >
> > > > > +static void update_batch_size(struct lru_gen_mm_walk *walk, struct folio *folio,
> > > > > +                           int old_gen, int new_gen)
> > > > > +{
> > > > > +     int type = folio_is_file_lru(folio);
> > > > > +     int zone = folio_zonenum(folio);
> > > > > +     int delta = folio_nr_pages(folio);
> > > > > +
> > > > > +     VM_BUG_ON(old_gen >= MAX_NR_GENS);
> > > > > +     VM_BUG_ON(new_gen >= MAX_NR_GENS);
> > > >
> > > > General rule: don't add new BUG_ONs, because they crash the kenrel.
> > > > It's better to use WARN_ON or WARN_ON_ONCE then try to figure out a way
> > > > to keep the kernel limping along.  At least so the poor user can gather logs.
> > >
> > > These are VM_BUG_ONs, which are BUILD_BUG_ONs except for (mostly MM) developers.
> >
> > I'm told that many production builds enable runtime VM_BUG_ONning.
> 
> Nobody wants to debug VM in production. Some distros that offer both
> the latest/LTS kernels do enable CONFIG_DEBUG_VM in the former so the
> latter can have better test coverage when it becomes available. Do
> people use the former in production? Absolutely, otherwise we won't
> have enough test coverage. Are we supposed to avoid CONFIG_DEBUG_VM? I
> don't think so, because it defeats the purpose of those distros
> enabling it in the first place.
> 
> The bottomline is that none of RHEL 8.5, SLES 15, Debian 11 enables
> CONFIG_DEBUG_VM.

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 

hp2:/home/akpm/yy> grep "DEBUG_VM[= ]" *.config 
kernel-aarch64-debug-fedora.config:CONFIG_DEBUG_VM=y
kernel-aarch64-debug-rhel.config:# CONFIG_DEBUG_VM is not set
kernel-aarch64-fedora.config:CONFIG_DEBUG_VM=y
kernel-aarch64-rhel.config:# CONFIG_DEBUG_VM is not set
kernel-armv7hl-debug-fedora.config:CONFIG_DEBUG_VM=y
kernel-armv7hl-fedora.config:CONFIG_DEBUG_VM=y
kernel-armv7hl-lpae-debug-fedora.config:CONFIG_DEBUG_VM=y
kernel-armv7hl-lpae-fedora.config:CONFIG_DEBUG_VM=y
kernel-ppc64le-debug-fedora.config:CONFIG_DEBUG_VM=y
kernel-ppc64le-debug-rhel.config:CONFIG_DEBUG_VM=y
kernel-ppc64le-fedora.config:CONFIG_DEBUG_VM=y
kernel-ppc64le-rhel.config:# CONFIG_DEBUG_VM is not set
kernel-s390x-debug-fedora.config:CONFIG_DEBUG_VM=y
kernel-s390x-debug-rhel.config:CONFIG_DEBUG_VM=y
kernel-s390x-fedora.config:CONFIG_DEBUG_VM=y
kernel-s390x-rhel.config:# CONFIG_DEBUG_VM is not set
kernel-s390x-zfcpdump-rhel.config:# CONFIG_DEBUG_VM is not set
kernel-x86_64-debug-fedora.config:CONFIG_DEBUG_VM=y
kernel-x86_64-debug-rhel.config:CONFIG_DEBUG_VM=y
kernel-x86_64-fedora.config:CONFIG_DEBUG_VM=y
kernel-x86_64-rhel.config:# CONFIG_DEBUG_VM is not set