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

[Yu Zhao]

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