linux-kernel - Re: [PATCH] mm: folio_zero_user: open code range computation in folio_zero

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Message-ID: <87pl6uei6l.fsf@oracle.com>
Date: Tue, 27 Jan 2026 15:42:42 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
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, tglx@...utronix.de, willy@...radead.org,
        raghavendra.kt@....com, chleroy@...nel.org, ioworker0@...il.com,
        lizhe.67@...edance.com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] mm: folio_zero_user: open code range computation in
 folio_zero_user()


David Hildenbrand (Red Hat) <david@...nel.org> writes:

> On 1/26/26 19:32, Ankur Arora wrote:
>> riscv64-gcc-linux-gnu (v8.5) reports a compile time assert in:
>>     r[2] = DEFINE_RANGE(clamp_t(s64, fault_idx - radius, pg.start, pg.end),
>>   		       clamp_t(s64, fault_idx + radius, pg.start, pg.end));
>> where it decides that pg.start > pg.end in:
>>    clamp_t(s64, fault_idx + radius, pg.start, pg.end));
>> where pg comes from:
>>    const struct range pg = DEFINE_RANGE(0, folio_nr_pages(folio) - 1);
>> That does not seem like it could be true. Even for pg.start == pg.end,
>> we would need folio_test_large() to evaluate to false at compile time:
>>    static inline unsigned long folio_nr_pages(const struct folio *folio)
>>    {
>> 	if (!folio_test_large(folio))
>> 		return 1;
>> 	return folio_large_nr_pages(folio);
>>    }
>> Workaround by open coding the range computation. Also, simplify the type
>> declarations for the relevant variables.
>> Reported-by: kernel test robot <lkp@...el.com>
>> Closes: https://lore.kernel.org/oe-kbuild-all/202601240453.QCjgGdJa-lkp@intel.com/
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>> ---
>> Hi Andrew
>> I'm not certain about linux-next rebasing protocol, but I'm guessing
>> this patch will be squashed in patch-8 ("mm: folio_zero_user: cache
>> neighbouring pages").
>> The commit message doesn't contain anything needing preserving if it is.
>> Thanks
>> Ankur
>>   mm/memory.c | 23 +++++++++++------------
>>   1 file changed, 11 insertions(+), 12 deletions(-)
>> diff --git a/mm/memory.c b/mm/memory.c
>> index ce933ee4a3dd..e49340f51fa9 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -7282,30 +7282,29 @@ static void clear_contig_highpages(struct page *page, unsigned long addr,
>>   void folio_zero_user(struct folio *folio, unsigned long addr_hint)
>>   {
>>   	const unsigned long base_addr = ALIGN_DOWN(addr_hint, folio_size(folio));
>> -	const long fault_idx = (addr_hint - base_addr) / PAGE_SIZE;
>>   	const struct range pg = DEFINE_RANGE(0, folio_nr_pages(folio) - 1);
>> -	const int radius = FOLIO_ZERO_LOCALITY_RADIUS;
>> +	const long fault_idx = (addr_hint - base_addr) / PAGE_SIZE;
>> +	const long radius = FOLIO_ZERO_LOCALITY_RADIUS;
>>   	struct range r[3];
>>   	int i;
>>     	/*
>> -	 * Faulting page and its immediate neighbourhood. Will be cleared at the
>> -	 * end to keep its cachelines hot.
>> +	 * Faulting page and its immediate neighbourhood. Cleared at the end to
>> +	 * keep its cachelines hot.
>>   	 */
>
> Why are there rather unrelated changes in this patch? Like this comment change,
> or the movement of "fualt_idx" declaration above?

Yeah, that was a mistake.

--
ankur