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

[Barry Song]

On Sun, Feb 25, 2024 at 9:54 AM SeongJae Park <sj@xxxxxxxxxx> wrote:
>
> On Sun, 25 Feb 2024 04:01:40 +0800 Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> > On Sun, Feb 25, 2024 at 3:07 AM SeongJae Park <sj@xxxxxxxxxx> wrote:
> > >
> > > Hi Barry,
> > >
> > > On Sat, 24 Feb 2024 12:37:59 +0800 Barry Song <21cnbao@xxxxxxxxx> 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?
>
> Thank you for checking this again.  You're right.
>
> Technically speaking, DAMON's usage of MADV_PAGEOUT is in vaddr.c.  paddrc is
> using not MADV_PAGEOUT but reclaim_pages().  Usage of reclaim_pages() from
> paddr is different from that of MADV_PAGEOUT since paddr doesn't clear PTE.  I
> was confused from the difference between vaddr and paddr.  I actually wanted to
> document the difference but haven't had a time for that yet.  Thank you for
> letting me remind this.

Hi SeongJae,

thanks! I bravely had a go at fixing the damon's doc[1]. as it seems
the fix is anyway needed
no matter if we have my patch to optimize MADV_PAGEOUT.

[1] https://lore.kernel.org/linux-mm/20240224215023.5271-1-21cnbao@xxxxxxxxx/

>
> So, your change on MADV_PAGEOUT will make an effect to vaddr, and I think it's
> ok.  Your change on reclaim_pages() could make an effect to paddr, depending on
> the additional parameter's value.  I now think it would better to make no
> effect here.  That is, let's keep the change for paddr.c in your patch as is.

thanks! it seems everything is quite clear now.

>
>
> Thanks,
> SJ
>
> >
> > >
> > >
> > > Thanks,
> > > SJ
> > >
> >

Thanks
Barry