linux-kernel - Re: [PATCH v7 11/16] mm/highmem: introduce clear_user

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Message-ID: <87ecrwewn3.fsf@oracle.com>
Date: Tue, 23 Sep 2025 13:34:08 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, 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 11/16] mm/highmem: introduce clear_user_highpages()


David Hildenbrand <david@...hat.com> writes:

> On 17.09.25 17:24, Ankur Arora wrote:
>> Define clear_user_highpages() which clears pages sequentially using
>> the single page variant.
>> With !CONFIG_HIGHMEM, pages are contiguous so use the range clearing
>> primitive clear_user_pages().
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>> ---
>>   include/linux/highmem.h | 18 ++++++++++++++++++
>>   1 file changed, 18 insertions(+)
>> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
>> index 6234f316468c..ed609987e24d 100644
>> --- a/include/linux/highmem.h
>> +++ b/include/linux/highmem.h
>> @@ -207,6 +207,24 @@ static inline void clear_user_highpage(struct page *page, unsigned long vaddr)
>>   }
>>   #endif
>>   +#ifndef clear_user_highpages
>
> Maybe we can add a simple kernel doc that points at the doc of clear_user_pages,
> but makes it clear that this for pages that might reside in highmem.

Didn't add one because clear_user_highpage() didn't have one. Will add
for both.

>> +static inline void clear_user_highpages(struct page *page, unsigned long vaddr,
>> +					unsigned int npages)
>> +{
>> +	if (!IS_ENABLED(CONFIG_HIGHMEM)) {
>> +		clear_user_pages(base, vaddr, page, npages);
>
> Single line should work
>
> 		clear_user_pages(page_address(page), vaddr, page, npages);

Unfortunately not. The problem is that I'm defining the fallback version
of clear_user_pages() as a macro in the previous patch.

        +#define clear_user_pages(addr, vaddr, pg, npages)	\
        +do {							\
        +	clear_user_page(addr, vaddr, pg);		\
        +	addr += PAGE_SIZE;				\
        +	vaddr += PAGE_SIZE;				\
        +	pg++;						\
        +} while (--npages)


And so using page_address() directly doesn't work because addr needs
to be an lvalue for the addition which page_address(page) isn't.


Thanks

--
ankur