linux-kernel - Re: [PATCH RFC 03/14] mm: compare pfns only if the entry is present when inserting pfns/pages

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Message-ID: <aFVhvYbRH2dtiFKY@localhost.localdomain>
Date: Fri, 20 Jun 2025 15:27:25 +0200
From: Oscar Salvador <osalvador@...e.de>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, nvdimm@...ts.linux.dev,
	Andrew Morton <akpm@...ux-foundation.org>,
	Juergen Gross <jgross@...e.com>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Alistair Popple <apopple@...dia.com>,
	Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Zi Yan <ziy@...dia.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
	Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Michal Hocko <mhocko@...e.com>, Jann Horn <jannh@...gle.com>,
	Pedro Falcato <pfalcato@...e.de>
Subject: Re: [PATCH RFC 03/14] mm: compare pfns only if the entry is present
 when inserting pfns/pages

On Tue, Jun 17, 2025 at 05:43:34PM +0200, David Hildenbrand wrote:
> Doing a pte_pfn() etc. of something that is not a present page table
> entry is wrong. Let's check in all relevant cases where we want to
> upgrade write permissions when inserting pfns/pages whether the entry
> is actually present.

Maybe I would add that's because the pte can have other info like
marker, swp_entry etc.

> It's not expected to have caused real harm in practice, so this is more a
> cleanup than a fix for something that would likely trigger in some
> weird circumstances.
> 
> At some point, we should likely unify the two pte handling paths,
> similar to how we did it for pmds/puds.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>

Reviewed-by: Oscar Salvador <osalvador@...e.de>

Should we scream if someone passes us a non-present entry?


> ---
>  mm/huge_memory.c | 4 ++--
>  mm/memory.c      | 4 ++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 8e0e3cfd9f223..e52360df87d15 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1392,7 +1392,7 @@ static int insert_pmd(struct vm_area_struct *vma, unsigned long addr,
>  		const unsigned long pfn = fop.is_folio ? folio_pfn(fop.folio) :
>  					  fop.pfn;
>  
> -		if (write) {
> +		if (write && pmd_present(*pmd)) {
>  			if (pmd_pfn(*pmd) != pfn) {
>  				WARN_ON_ONCE(!is_huge_zero_pmd(*pmd));
>  				return -EEXIST;
> @@ -1541,7 +1541,7 @@ static void insert_pud(struct vm_area_struct *vma, unsigned long addr,
>  		const unsigned long pfn = fop.is_folio ? folio_pfn(fop.folio) :
>  					  fop.pfn;
>  
> -		if (write) {
> +		if (write && pud_present(*pud)) {
>  			if (WARN_ON_ONCE(pud_pfn(*pud) != pfn))
>  				return;
>  			entry = pud_mkyoung(*pud);
> diff --git a/mm/memory.c b/mm/memory.c
> index a1b5575db52ac..9a1acd057ce59 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2137,7 +2137,7 @@ static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
>  	pte_t pteval = ptep_get(pte);
>  
>  	if (!pte_none(pteval)) {
> -		if (!mkwrite)
> +		if (!mkwrite || !pte_present(pteval))
>  			return -EBUSY;

Why EBUSY? because it might transitory?


-- 
Oscar Salvador
SUSE Labs