linux-kernel - Re: [PATCH v1 2/2] mm/damon: Add damos

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Message-ID: <aIqbpFsoqbt77j8f@devbig569.cln6.facebook.com>
Date: Wed, 30 Jul 2025 15:24:36 -0700
From: Yueyang Pan <pyyjason@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: SeongJae Park <sj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Usama Arif <usamaarif642@...il.com>, damon@...ts.linux.dev,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] mm/damon: Add damos_stat support for vaddr

On Tue, Jul 29, 2025 at 04:11:32PM +0200, David Hildenbrand wrote:
> On 29.07.25 15:53, Yueyang Pan wrote:
> > From: PanJason <pyyjason@...il.com>
> > 
> > This patch adds support for damos_stat in virtual address space.
> > It leverages the walk_page_range to walk the page table and gets
> > the folio from page table. The last folio scanned is stored in
> > damos->last_applied to prevent double counting.
> > ---
> >   mm/damon/vaddr.c | 113 ++++++++++++++++++++++++++++++++++++++++++++++-
> >   1 file changed, 112 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> > index 87e825349bdf..3e319b51cfd4 100644
> > --- a/mm/damon/vaddr.c
> > +++ b/mm/damon/vaddr.c
> > @@ -890,6 +890,117 @@ static unsigned long damos_va_migrate(struct damon_target *target,
> >   	return applied * PAGE_SIZE;
> >   }
> > +struct damos_va_stat_private {
> > +	struct damos *scheme;
> > +	unsigned long *sz_filter_passed;
> > +};
> > +
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +static int damos_va_stat_pmd_entry(pmd_t *pmd, unsigned long addr,
> > +		unsigned long next, struct mm_walk *walk)
> > +{
> > +	struct damos_va_stat_private *priv = walk->private;
> > +	struct damos *s = priv->scheme;
> > +	unsigned long *sz_filter_passed = priv->sz_filter_passed;
> > +	struct folio *folio;
> > +	spinlock_t *ptl;
> > +	pmd_t pmde;
> > +
> > +	ptl = pmd_lock(walk->mm, pmd);
> > +	pmde = pmdp_get(pmd);
> > +
> > +	if (!pmd_present(pmde) || !pmd_trans_huge(pmde))
> > +		goto unlock;
> > +
> > +	/* Tell page walk code to not split the PMD */
> > +	walk->action = ACTION_CONTINUE;
> > +
> > +	folio = damon_get_folio(pmd_pfn(pmde));
> > +	if (!folio)
> > +		goto unlock;
> > +
> > +	if (damon_invalid_damos_folio(folio, s))
> > +		goto update_last_applied;
> > +
> > +	if (!damos_va_filter_out(s, folio, walk->vma, addr, NULL, pmd)){
> > +		*sz_filter_passed += folio_size(folio);
> 
> See my comment below regarding vm_normal_page and folio references.
> 
> But this split into two handlers is fairly odd. Usually we only have a
> pmd_entry callback (see madvise_cold_or_pageout_pte_range as an example),
> and handle !CONFIG_TRANSPARENT_HUGEPAGE in there.
> 
> Then, there is also no need to mess with ACTION_CONTINUE
> 

Hi David. Thanks for your comment. I was not aware of the convention so I
followed the existing code. I had a discussion with SJ today and in the
next version I will change this by combining *pmd_entry() and *pte_entry().
We will also format the existing code in future patches.

> > +	}
> > +
> > +	folio_put(folio);
> > +update_last_applied:
> > +	s->last_applied = folio;
> > +unlock:
> > +	spin_unlock(ptl);
> > +	return 0;
> > +}
> > +#else
> > +#define damon_va_stat_pmd_entry NULL
> > +#endif
> > +
> > +static int damos_va_stat_pte_entry(pte_t *pte, unsigned long addr,
> > +		unsigned long next, struct mm_walk *walk)
> > +{
> > +	struct damos_va_stat_private *priv = walk->private;
> > +	struct damos *s = priv->scheme;
> > +	unsigned long *sz_filter_passed = priv->sz_filter_passed;
> > +	struct folio *folio;
> > +	pte_t ptent;
> > +
> > +	ptent = ptep_get(pte);
> > +	if (pte_none(ptent) || !pte_present(ptent))
> > +		return 0;
> > +
> > +	folio = damon_get_folio(pte_pfn(ptent));
> > +	if (!folio)
> > +		return 0;
> 
> We have vm_normal_folio() and friends for a reason -- so you don't have to
> do pte_pfn() manually.
> 
> ... and now I am confused. We are holding the PTL, so why would you have to
> grab+put a folio reference here *at all*.
> 

Thanks for pointing out. I thought someone could still change the folio and
realized it could not happen with PTL. I will use vm_normal_folio* in the
next version.

> -- 
> Cheers,
> 
> David / dhildenb
>