linux-kernel - Re: [PATCH 2/2] mm: convert do_set

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Message-ID: <aBtNrQNlL7hjLrTZ@casper.infradead.org>
Date: Wed, 7 May 2025 13:10:21 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, hannes@...xchg.org,
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com,
	npache@...hat.com, ryan.roberts@....com, dev.jain@....com,
	ziy@...dia.com, vbabka@...e.cz, rppt@...nel.org, surenb@...gle.com,
	mhocko@...e.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: convert do_set_pmd() to take a folio

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.