Re: [PATCH] mm/khugepaged: Do not fail collapse_pte_mapped_thp() on SCAN_PMD_NULL

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, Sep 12, 2025 at 05:58:11PM +0100, Kiryl Shutsemau wrote:
> From: Kiryl Shutsemau <kas@kernel.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@kernel.org>

LGTM, nice!

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

> ---
>  mm/khugepaged.c | 25 ++++++++++++++++++++++++-
>  1 file changed, 24 insertions(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b486c1d19b2d..9e76a4f46df9 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1488,6 +1488,28 @@ static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr,
>  	return SCAN_SUCCEED;
>  }
>
> +static int install_huge_pmd(struct vm_area_struct *vma, unsigned long haddr,
> +			    pmd_t *pmd, struct folio *folio)
> +{
> +	struct mm_struct *mm = vma->vm_mm;
> +	pgd_t *pgd;
> +	p4d_t *p4d;
> +	pud_t *pud;
> +
> +	pgd = pgd_offset(mm, haddr);
> +	p4d = p4d_alloc(mm, pgd, haddr);
> +	if (!p4d)
> +		return SCAN_FAIL;
> +	pud = pud_alloc(mm, p4d, haddr);
> +	if (!pud)
> +		return SCAN_FAIL;
> +	pmd = pmd_alloc(mm, pud, haddr);
> +	if (!pmd)
> +		return SCAN_FAIL;
> +
> +	return set_huge_pmd(vma, haddr, pmd, folio, &folio->page);
> +}
> +
>  /**
>   * collapse_pte_mapped_thp - Try to collapse a pte-mapped THP for mm at
>   * address haddr.
> @@ -1556,6 +1578,7 @@ 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.
> @@ -1700,7 +1723,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>  maybe_install_pmd:
>  	/* step 5: install pmd entry */
>  	result = install_pmd
> -			? set_huge_pmd(vma, haddr, pmd, folio, &folio->page)
> +			? install_huge_pmd(vma, haddr, pmd, folio)
>  			: SCAN_SUCCEED;
>  	goto drop_folio;
>  abort:
> --
> 2.50.1