linux-kernel - Re: [PATCHv2] mm/khugepaged: Do not fail collapse_pte_mapped_thp() on SCAN_PMD

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Message-ID: <ecac1e7f-1709-4348-b8ff-fa1c0f087548@lucifer.local>
Date: Tue, 16 Sep 2025 10:54:12 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Kiryl Shutsemau <kirill@...temov.name>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>, Zi Yan <ziy@...dia.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2] mm/khugepaged: Do not fail collapse_pte_mapped_thp()
 on SCAN_PMD_NULL

On Mon, Sep 15, 2025 at 02:52:53PM +0100, Kiryl Shutsemau wrote:
> From: Kiryl Shutsemau <kas@...nel.org>
>
> MADV_COLLAPSE on a file mapping behaves inconsistently depending on if
> PMD page table is installed or not.
>
> Consider following example:
>
> 	p = mmap(NULL, 2UL << 20, PROT_READ | PROT_WRITE,
> 		 MAP_SHARED, fd, 0);
> 	err = madvise(p, 2UL << 20, MADV_COLLAPSE);
>
> fd is a populated tmpfs file.
>
> The result depends on the address that the kernel returns on mmap().
> If it is located in an existing PMD table, the madvise() will succeed.
> However, if the table does not exist, it will fail with -EINVAL.
>
> This occurs because find_pmd_or_thp_or_none() returns SCAN_PMD_NULL when
> a page table is missing, which causes collapse_pte_mapped_thp() to fail.
>
> SCAN_PMD_NULL and SCAN_PMD_NONE should be treated the same in
> collapse_pte_mapped_thp(): install the PMD leaf entry and allocate page
> tables as needed.
>
> Signed-off-by: Kiryl Shutsemau <kas@...nel.org>

There was a v1 with tags, you've not propagated any of them? Did you feel
the change was enough to remove them?

Anyway, LGTM so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

> ---
>
> v2:
>  - Modify set_huge_pmd() instead of introducing install_huge_pmd();
>
> ---
>  mm/khugepaged.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b486c1d19b2d..986718599355 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1472,15 +1472,32 @@ static void collect_mm_slot(struct khugepaged_mm_slot *mm_slot)
>  static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr,
>  			pmd_t *pmdp, struct folio *folio, struct page *page)
>  {
> +	struct mm_struct *mm = vma->vm_mm;
>  	struct vm_fault vmf = {
>  		.vma = vma,
>  		.address = addr,
>  		.flags = 0,
> -		.pmd = pmdp,
>  	};
> +	pgd_t *pgdp;
> +	p4d_t *p4dp;
> +	pud_t *pudp;
>
>  	mmap_assert_locked(vma->vm_mm);

NIT: you have mm as a local var should use here too. Not a big deal though
obviously...

>
> +	if (!pmdp) {
> +		pgdp = pgd_offset(mm, addr);
> +		p4dp = p4d_alloc(mm, pgdp, addr);
> +		if (!p4dp)
> +			return SCAN_FAIL;
> +		pudp = pud_alloc(mm, p4dp, addr);
> +		if (!pudp)
> +			return SCAN_FAIL;
> +		pmdp = pmd_alloc(mm, pudp, addr);
> +		if (!pmdp)
> +			return SCAN_FAIL;
> +	}
> +
> +	vmf.pmd = pmdp;
>  	if (do_set_pmd(&vmf, folio, page))
>  		return SCAN_FAIL;
>
> @@ -1556,6 +1573,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>  	switch (result) {
>  	case SCAN_SUCCEED:
>  		break;
> +	case SCAN_PMD_NULL:
>  	case SCAN_PMD_NONE:
>  		/*
>  		 * All pte entries have been removed and pmd cleared.
> --
> 2.50.1
>