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

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Message-ID: <0cb3d5a5-683b-4dba-90a8-b45ab83eec53@redhat.com>
Date: Tue, 29 Jul 2025 16:11:32 +0200
From: David Hildenbrand <david@...hat.com>
To: Yueyang Pan <pyyjason@...il.com>, SeongJae Park <sj@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Usama Arif <usamaarif642@...il.com>
Cc: 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 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

> +	}
> +
> +	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*.

-- 
Cheers,

David / dhildenb