linux-kernel - Re: [PATCH] mm: fix apply_to_existing_page

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Message-ID: <00a37819-b2ae-4399-b21d-86339059c46a@redhat.com>
Date: Wed, 9 Apr 2025 11:52:43 +0200
From: David Hildenbrand <david@...hat.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Daniel Axtens <dja@...ens.net>
Subject: Re: [PATCH] mm: fix apply_to_existing_page_range()

On 09.04.25 11:40, Kirill A. Shutemov wrote:
> In the case of apply_to_existing_page_range(), apply_to_pte_range() is
> reached with 'create' set to false. When !create, the loop over the PTE
> page table is broken.
> 
> apply_to_pte_range() will only move to the next PTE entry if 'create' is
> true or if the current entry is not pte_none().
> 
> This means that the user of apply_to_existing_page_range() will not have
> 'fn' called for any entries after the first pte_none() in the PTE page
> table.
> 
> Fix the loop logic in apply_to_pte_range().
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Fixes: be1db4753ee6 ("mm/memory.c: add apply_to_existing_page_range() helper")
> Cc: Daniel Axtens <dja@...ens.net>
> ---
>   mm/memory.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index fb7b8dc75167..2094564f4dfb 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2907,11 +2907,11 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
>   	if (fn) {
>   		do {
>   			if (create || !pte_none(ptep_get(pte))) {
> -				err = fn(pte++, addr, data);
> +				err = fn(pte, addr, data);
>   				if (err)
>   					break;
>   			}
> -		} while (addr += PAGE_SIZE, addr != end);
> +		} while (pte++, addr += PAGE_SIZE, addr != end);
>   	}
>   	*mask |= PGTBL_PTE_MODIFIED;
>   

LGTM. just curious, did you run into any actual issues that are worth 
describing?

It should affect apply_to_existing_page_range() users where 
create==false. There are not many, and likely most PTEs in the range 
they are passing are all non-none.

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb