Re: [PATCH v2 1/4] mm: mincore: use walk_page_range_vma() in do_mincore()

[Pedro Falcato <pfalcato@suse.de>]

Please CC reviewers properly!

On Thu, Jun 18, 2026 at 05:28:42PM +0800, Kefeng Wang wrote:
> The do_mincore() uses walk_page_range() to walk the page table.
> Fortunately, the caller always passes start/end that falls within
> a single VMA, so it's safe to use the walk_page_range_vma() in
> do_mincore() to eliminate an unnecessary find_vma() lookup.
> 
> Unlike walk_page_range(), walk_page_range_vma() does not call
> walk_page_test(), which handles VM_PFNMAP by invoking ->pte_hole()

Why not? Can we fix that instead? I really don't like having this open
coded in callers. Are there callers of walk_page_range_vma() that expect
to look at PFNMAP mappings as well? From what I can see, the callers all
seem to operate on folios (and/or anonymous memory).

> to skip the page table walk. Without this check, PFNMAP PTEs
> would be treated as present by mincore_pte_range(), changing
> the returned residency status. Handle VM_PFNMAP explicitly in
> do_mincore() to preserve the original behavior.
> 
> Acked-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
>  mm/mincore.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/mincore.c b/mm/mincore.c
> index 296f2e3922b5..0c6731ae6c4d 100644
> --- a/mm/mincore.c
> +++ b/mm/mincore.c
> @@ -259,7 +259,21 @@ static long do_mincore(unsigned long addr, unsigned long pages, unsigned char *v
>  		memset(vec, 1, pages);
>  		return pages;
>  	}
> -	err = walk_page_range(vma->vm_mm, addr, end, &mincore_walk_ops, vec);
> +
> +	/*
> +	 * walk_page_range_vma() does not call walk_page_test(), which
> +	 * handles VM_PFNMAP VMA by invoking ->pte_hole() to skip the
> +	 * page table walk. Without this check, PFNMAP PTEs would be
> +	 * treated as present by mincore_pte_range(), changing the returned
> +	 * residency status from the historical "not resident" to "resident".
> +	 * Handle VM_PFNMAP explicitly to preserve the original behavior.
> +	 */

This whole comment looks poised to rot very very quickly.

> +	if (vma->vm_flags & VM_PFNMAP) {
> +		__mincore_unmapped_range(addr, end, vma, vec);
> +		return (end - addr) >> PAGE_SHIFT;
> +	}
> +
> +	err = walk_page_range_vma(vma, addr, end, &mincore_walk_ops, vec);
>  	if (err < 0)
>  		return err;
>  	return (end - addr) >> PAGE_SHIFT;
> -- 
> 2.27.0
> 
> 
> 

-- 
Pedro