Re: [PATCH 2/2] mm: convert do_set_pmd() to take a folio

[Matthew Wilcox <willy@infradead.org>]

On Wed, May 07, 2025 at 05:26:13PM +0800, Baolin Wang wrote:
> In do_set_pmd(), we always use the folio->page to build PMD mappings for
> the entire folio. Since all callers of do_set_pmd() already hold a stable
> folio, converting do_set_pmd() to take a folio is safe and more straightforward.

What testing did you do of this?

> -vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
> +vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio)
>  {
> -	struct folio *folio = page_folio(page);
>  	struct vm_area_struct *vma = vmf->vma;
>  	bool write = vmf->flags & FAULT_FLAG_WRITE;
>  	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
>  	pmd_t entry;
>  	vm_fault_t ret = VM_FAULT_FALLBACK;
> +	struct page *page;

Because I see nowhere in this patch that you initialise 'page'.

And that's really the important part.  You seem to be assuming that a
folio will never be larger than PMD size, and I'm not comfortable with
that assumption.  It's a limitation I put in place a few years ago so we
didn't have to find and fix all those assumptions immediately, but I
imagine that some day we'll want to have larger folios.

So unless you can derive _which_ page in the folio we want to map from
the vmf, NACK this patch.