Re: [PATCH RFC] mm: madvise: pageout: ignore references rather than clearing young

[Barry Song <21cnbao@gmail.com>]

On Sun, Feb 25, 2024 at 3:07 AM SeongJae Park <sj@kernel.org> wrote:
>
> Hi Barry,
>
> On Sat, 24 Feb 2024 12:37:59 +0800 Barry Song <21cnbao@gmail.com> wrote:
>
> [...]
> >
> > BTW，
> > Hi SeongJae,
> > I am not quite sure if damon also needs this, so I have kept damon as is by
> > setting ignore_references = false.  MADV_PAGEOUT is an explicit hint users
> > don't want the memory to be reclaimed, I don't know if it is true for damon as
> > well. If you have some comments, please chime in.
>
> Thank you for calling my name :)
>
> For DAMON's usecase, the document simply says the behavior would be same to
> MADV_PAGEOUT, so if we conclude to change MADV_PAGEOUT, I think same change
> should be made for DAMON's usecase, or update DAMON document.

Hi SeongJae,

I don't find similar clearing pte young in damon_pa_pageout(), so i
guess damon's
behaviour is actually different with MADV_PAGEOUT which has  pte-clearing. damon
is probably the best-effort but MADV_PAGEOUT isn't .

static unsigned long damon_pa_pageout(struct damon_region *r, struct damos *s)
{
        unsigned long addr, applied;
        LIST_HEAD(folio_list);

        for (addr = r->ar.start; addr < r->ar.end; addr += PAGE_SIZE) {
                     struct folio *folio = damon_get_folio(PHYS_PFN(addr));
                     ....

                     if (damos_pa_filter_out(s, folio))
                                goto put_folio;

                    folio_clear_referenced(folio);
                    folio_test_clear_young(folio);
                    if (!folio_isolate_lru(folio))
                         goto put_folio;
                    if (folio_test_unevictable(folio))
                          folio_putback_lru(folio);
                    else
                          list_add(&folio->lru, &folio_list);
put_folio:
               folio_put(folio);
         }
         applied = reclaim_pages(&folio_list);
         cond_resched();
         return applied * PAGE_SIZE;
}

am i missing something?

>
>
> Thanks,
> SJ
>

Thanks
Barry