linux-kernel - Re: [PATCH v7 12/16] arm: mm: define clear_user

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Message-ID: <d03dd7ab-5237-4de6-8872-a8ae2e9b7c5d@redhat.com>
Date: Wed, 24 Sep 2025 13:10:32 +0200
From: David Hildenbrand <david@...hat.com>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
 akpm@...ux-foundation.org, bp@...en8.de, dave.hansen@...ux.intel.com,
 hpa@...or.com, mingo@...hat.com, mjguzik@...il.com, luto@...nel.org,
 peterz@...radead.org, acme@...nel.org, namhyung@...nel.org,
 tglx@...utronix.de, willy@...radead.org, raghavendra.kt@....com,
 boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v7 12/16] arm: mm: define clear_user_highpages()

On 24.09.25 00:25, Ankur Arora wrote:
> 
> David Hildenbrand <david@...hat.com> writes:
> 
>> On 17.09.25 17:24, Ankur Arora wrote:
>>> For configurations with CONFIG_MMU we do not define clear_user_page().
>>> This runs into issues for configurations with !CONFIG_HIGHMEM, because
>>> clear_user_highpages() expects to clear_user_page() (via a default
>>> version of clear_user_pages()).
>>
>> I'm confused. Can you elaborate once more why we cannot take care of that in
>> common code?
> 
> So my definition of clear_user_highpages,
> 
>      +#ifndef clear_user_highpages
>      +static inline void clear_user_highpages(struct page *page, unsigned long vaddr,
>      +					unsigned int npages)
>      +{
>      +	if (!IS_ENABLED(CONFIG_HIGHMEM)) {
>      +		void *base = page_address(page);
>      +		clear_user_pages(base, vaddr, page, npages);
>      +		return;
>      +	}
>      +
>      +	do {
>      +		clear_user_highpage(page, vaddr);
>      +		vaddr += PAGE_SIZE;
>      +		page++;
>      +	} while (--npages);
>      +}
>      +#endif
> 
> assumes one of the following:
> 
>    1. clear_user_highpages is defined by the architecture or,
>    2. HIGHMEM => arch defines clear_user_highpage or clear_user_page
>    3. !HIGHMEM => arch defines clear_user_pages or clear_user_page
> 
> Case 2 is fine, since ARM has clear_user_highpage().
> 
> Case 3 runs into a problem since ARM doesn't have clear_user_pages()
> or clear_user_page() (it does have the second, but only with !CONFIG_MMU).

I think we should look into having a generic fallback version in common 
code instead for that case, and not require the arch to implement such a 
loop around clear_user_highpage().

-- 
Cheers

David / dhildenb