Re: [PATCHv2] mm/khugepaged: Do not fail collapse_pte_mapped_thp() on SCAN_PMD_NULL

[Dev Jain <dev.jain@arm.com>]

On 15/09/25 7:22 pm, 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>
> ---
>
> 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);

I was going to reply to v1 - you could replace vma->vm_mm
with mm here.

Reviewed-by: Dev Jain <dev.jain@arm.com>