Re: [PATCHv4] mm: skip CMA pages when they are not available

[Zhaoyang Huang <huangzhaoyang@gmail.com>]

On Fri, May 26, 2023 at 4:03 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Mon, May 22, 2023 at 02:36:03PM +0800, zhaoyang.huang wrote:
> > +#ifdef CONFIG_CMA
> > +/*
> > + * It is waste of effort to scan and reclaim CMA pages if it is not available
> > + * for current allocation context
> > + */
> > +static bool skip_cma(struct folio *folio, struct scan_control *sc)
> > +{
> > +     if (!current_is_kswapd() &&
> > +                     gfp_migratetype(sc->gfp_mask) != MIGRATE_MOVABLE &&
> > +                     get_pageblock_migratetype(&folio->page) == MIGRATE_CMA)
> > +             return true;
> > +     return false;
> > +}
> > +#else
> > +static bool skip_cma(struct folio *folio, struct scan_control *sc)
> > +{
> > +     return false;
> > +}
> > +#endif
> > +
> >  /*
> >   * Isolating page from the lruvec to fill in @dst list by nr_to_scan times.
> >   *
> > @@ -2239,7 +2259,8 @@ static unsigned long isolate_lru_folios(unsigned long nr_to_scan,
> >               nr_pages = folio_nr_pages(folio);
> >               total_scan += nr_pages;
> >
> > -             if (folio_zonenum(folio) > sc->reclaim_idx) {
> > +             if (folio_zonenum(folio) > sc->reclaim_idx ||
> > +                             skip_cma(folio, sc)) {
> >                       nr_skipped[folio_zonenum(folio)] += nr_pages;
> >                       move_to = &folios_skipped;
> >                       goto move;
>
> I have no idea if what this patch is trying to accomplish is correct,
> but I no longer object to how it is doing it.
IMO, this is necessary as there could be such weird scenario, that is
an GFP_KERNEL allocation might get 32 MIGRATE_CMA pages via
direct_reclaim which lead to a low PSI_MEM/vmpressure value but return
a NULL pointer