linux-kernel - Re: [PATCH 1/1] mm/memory: Fix boundary check for next PFN in folio_pte

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Date: Tue, 27 Feb 2024 08:30:38 +0100
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>, akpm@...ux-foundation.org
Cc: ryan.roberts@....com, 21cnbao@...il.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/memory: Fix boundary check for next PFN in
 folio_pte_batch()

On 27.02.24 08:04, Lance Yang wrote:
> Previously, in folio_pte_batch(), only the upper boundary of the
> folio was checked using '>=' for comparison. This led to
> incorrect behavior when the next PFN exceeded the lower boundary
> of the folio, especially in corner cases where the next PFN might
> fall into a different folio.

Which commit does this fix?

The introducing commit (f8d937761d65c87e9987b88ea7beb7bddc333a0e) is 
already in mm-stable, so we would need a Fixes: tag. Unless, Ryan's 
changes introduced a problem.

BUT

I don't see what is broken. :)

Can you please give an example/reproducer?

We know that the first PTE maps the folio. By incrementing the PFN using 
pte_next_pfn/pte_advance_pfn, we cannot suddenly get a lower PFN.

So how would pte_advance_pfn(folio_start_pfn + X) suddenly give us a PFN 
lower than folio_start_pfn?

Note that we are not really concerned about any kind of 
pte_advance_pfn() overflow that could generate PFN=0. I convinces myself 
that that that is something we don't have to worry about.

[I also thought about getting rid of the pte_pfn(pte) >= folio_end_pfn 
and instead limiting end_ptep. But that requires more work before the 
loop and feels more like a micro-optimization.]

> 
> Signed-off-by: Lance Yang <ioworker0@...il.com>
> ---
>   mm/memory.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 642b4f2be523..e5291d1e8c37 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -986,12 +986,15 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>   		pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
>   		bool *any_writable)
>   {
> -	unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
> +	unsigned long folio_start_pfn, folio_end_pfn;
>   	const pte_t *end_ptep = start_ptep + max_nr;
>   	pte_t expected_pte, *ptep;
>   	bool writable;
>   	int nr;
>   
> +	folio_start_pfn = folio_pfn(folio);
> +	folio_end_pfn = folio_start_pfn + folio_nr_pages(folio);
> +
>   	if (any_writable)
>   		*any_writable = false;
>   
> @@ -1015,7 +1018,7 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>   		 * corner cases the next PFN might fall into a different
>   		 * folio.
>   		 */
> -		if (pte_pfn(pte) >= folio_end_pfn)
> +		if (pte_pfn(pte) >= folio_end_pfn || pte_pfn(pte) < folio_start_pfn)
>   			break;
>   
>   		if (any_writable)

-- 
Cheers,

David / dhildenb