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Date:   Thu, 14 Apr 2022 18:56:54 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Yu Zhao <yuzhao@google.com>
Cc:     Stephen Rothwell <sfr@rothwell.id.au>,
        Linux-MM <linux-mm@kvack.org>, Andi Kleen <ak@linux.intel.com>,
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Subject: Re: [PATCH v10 08/14] mm: multi-gen LRU: support page table walks
Message-Id: <20220414185654.e7150bcbe859e0dd4b9c61af@linux-foundation.org>
In-Reply-To: <CAOUHufYeC=Kuu59BPL_48sM67CqACxH2wWy-SYGXpadgMDmY3w@mail.gmail.com>
References: <20220407031525.2368067-1-yuzhao@google.com>
        <20220407031525.2368067-9-yuzhao@google.com>
        <20220411191621.0378467ad99ebc822d5ad005@linux-foundation.org>
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On Thu, 14 Apr 2022 19:14:54 -0600 Yu Zhao <yuzhao@google.com> wrote:

> On Mon, Apr 11, 2022 at 8:16 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > On Wed,  6 Apr 2022 21:15:20 -0600 Yu Zhao <yuzhao@google.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.