linux-kernel - Re: [PATCH 28/45] mm/damon: Enable damon_mkold_pmd

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Message-ID: <b98fce2c-4ad5-4a4c-9ad8-b8b307078413@redhat.com>
Date: Thu, 4 Jul 2024 13:03:22 +0200
From: David Hildenbrand <david@...hat.com>
To: Oscar Salvador <osalvador@...e.de>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Peter Xu <peterx@...hat.com>, Muchun Song <muchun.song@...ux.dev>,
 SeongJae Park <sj@...nel.org>, Miaohe Lin <linmiaohe@...wei.com>,
 Michal Hocko <mhocko@...e.com>, Matthew Wilcox <willy@...radead.org>,
 Christophe Leroy <christophe.leroy@...roup.eu>
Subject: Re: [PATCH 28/45] mm/damon: Enable damon_mkold_pmd_entry to handle
 hugetlb vmas

On 04.07.24 06:31, Oscar Salvador wrote:
> PMD-mapped hugetlb vmas will also reach damon_mkold_pmd_entry.
> Add the required code so it knows how to handle those there.
> 
> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> ---
>   mm/damon/ops-common.c | 21 ++++++++++++++++-----
>   mm/damon/vaddr.c      | 15 +++++----------
>   2 files changed, 21 insertions(+), 15 deletions(-)
> 

(besides a lot of this code needing cleanups and likely fixes)

> diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
> index d25d99cb5f2b..6727658a3ef5 100644
> --- a/mm/damon/ops-common.c
> +++ b/mm/damon/ops-common.c
> @@ -53,18 +53,29 @@ void damon_ptep_mkold(pte_t *pte, struct vm_area_struct *vma, unsigned long addr
>   
>   void damon_pmdp_mkold(pmd_t *pmd, struct vm_area_struct *vma, unsigned long addr)
>   {
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -	struct folio *folio = damon_get_folio(pmd_pfn(pmdp_get(pmd)));
> +#ifdef CONFIG_PGTABLE_HAS_HUGE_LEAVES
> +	struct folio *folio;
> +	unsigned long size;
> +
> +	if (is_vm_hugetlb_page(vma)) {
> +		folio = pfn_folio(pdm_pfn(*pmd))
> +		folio_get(folio);
> +		size = huge_page_size(hstate_vma(vma));
> +	} else {
> +		folio = damon_get_folio(pmd_pfn(*pmd));
> +		size = PMD_SIZE;
> +	}
>   
>   	if (!folio)
> -		return;
> +		return 0;
>   
> -	if (pmdp_clear_young_notify(vma, addr, pmd))
> +	if (pmdp_test_and_clear_young(vma, addr, pmd) ||
> +	    mmu_notifier_clear_young(mm, addr, addr + size))
>   		folio_set_young(folio);

And I think here is the issue for both the cont-PMD and cont-PTE case:

For hugetlb we *absolutely must* use the set_huge_pte_at()-style 
functions, otherwise we might suddenly lose the cont-pte/cont-pmd bit. 
We cannot arbitrarily replace these "huge_pte" functions by others that 
work on individual PTEs/PMDs.

(noting that the hugetlb code in damon_hugetlb_mkold() is likely not 
correct, because we could be losing concurrently set dirty bits I believe)

-- 
Cheers,

David / dhildenb