linux-kernel - Re: [PATCH v3 5/7] mm: Remember young/dirty bit for page migrations

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Date:   Mon, 15 Aug 2022 15:18:50 -0400
From:   Peter Xu <peterx@...hat.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Minchan Kim <minchan@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <andi.kleen@...el.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v3 5/7] mm: Remember young/dirty bit for page migrations

On Fri, Aug 12, 2022 at 10:32:48AM +0800, Huang, Ying wrote:
> Peter Xu <peterx@...hat.com> writes:
> 
> > On Tue, Aug 09, 2022 at 06:00:58PM -0400, Peter Xu wrote:
> >> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> >> index 27fb37d65476..699f821b8443 100644
> >> --- a/mm/migrate_device.c
> >> +++ b/mm/migrate_device.c
> >> @@ -221,6 +221,10 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> >>  			else
> >>  				entry = make_readable_migration_entry(
> >>  							page_to_pfn(page));
> >> +			if (pte_young(pte))
> >> +				entry = make_migration_entry_young(entry);
> >> +			if (pte_dirty(pte))
> >> +				entry = make_migration_entry_dirty(entry);
> >>  			swp_pte = swp_entry_to_pte(entry);
> >>  			if (pte_present(pte)) {
> >>  				if (pte_soft_dirty(pte))
> >
> > This change needs to be wrapped with pte_present() at least..
> >
> > I also just noticed that this change probably won't help anyway because:
> >
> >   (1) When ram->device, the pte will finally be replaced with a device
> >       private entry, and device private entry does not yet support A/D, it
> >       means A/D will be dropped again,
> >
> >   (2) When device->ram, we are missing information on either A/D bits, or
> >       even if device private entries start to suport A/D, it's still not
> >       clear whether we should take device read/write into considerations
> >       too on the page A/D bits to be accurate.
> >
> > I think I'll probably keep the code there for completeness, but I think it
> > won't really help much until more things are done.
> 
> It appears that there are more issues.  Between "pte = *ptep" and pte
> clear, CPU may set A/D bit in PTE, so we may need to update pte when
> clearing PTE.

Agreed, I didn't see it a huge problem with current code, but it should be
better in that way.

> And I don't find the TLB is flushed in some cases after PTE is cleared.

I think it's okay to not flush tlb if pte not present.  But maybe you're
talking about something else?

Thanks,

-- 
Peter Xu