linux-kernel - Re: [PATCH v1 4/5] mm: ptdump: Support hugepd table entries

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Date:   Fri, 16 Apr 2021 07:25:36 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Daniel Axtens <dja@...ens.net>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Steven Price <steven.price@....com>, akpm@...ux-foundation.org
Cc:     linux-arch@...r.kernel.org, linux-s390@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-riscv@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v1 4/5] mm: ptdump: Support hugepd table entries

Hi Daniel,

Le 16/04/2021 à 01:29, Daniel Axtens a écrit :
> Hi Christophe,
> 
>> Which hugepd, page table entries can be at any level
>> and can be of any size.
>>
>> Add support for them.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>> ---
>>   mm/ptdump.c | 17 +++++++++++++++--
>>   1 file changed, 15 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/ptdump.c b/mm/ptdump.c
>> index 61cd16afb1c8..6efdb8c15a7d 100644
>> --- a/mm/ptdump.c
>> +++ b/mm/ptdump.c
>> @@ -112,11 +112,24 @@ static int ptdump_pte_entry(pte_t *pte, unsigned long addr,
>>   {
>>   	struct ptdump_state *st = walk->private;
>>   	pte_t val = ptep_get(pte);
>> +	unsigned long page_size = next - addr;
>> +	int level;
>> +
>> +	if (page_size >= PGDIR_SIZE)
>> +		level = 0;
>> +	else if (page_size >= P4D_SIZE)
>> +		level = 1;
>> +	else if (page_size >= PUD_SIZE)
>> +		level = 2;
>> +	else if (page_size >= PMD_SIZE)
>> +		level = 3;
>> +	else
>> +		level = 4;
>>   
>>   	if (st->effective_prot)
>> -		st->effective_prot(st, 4, pte_val(val));
>> +		st->effective_prot(st, level, pte_val(val));
>>   
>> -	st->note_page(st, addr, 4, pte_val(val), PAGE_SIZE);
>> +	st->note_page(st, addr, level, pte_val(val), page_size);
> 
> It seems to me that passing both level and page_size is a bit redundant,
> but I guess it does reduce the impact on each arch's code?

Exactly, as shown above, the level can be re-calculated based on the page size, but it would be a 
unnecessary impact on all architectures and would duplicate the re-calculation of the level whereas 
in most cases we get it for free from the caller.

> 
> Kind regards,
> Daniel
> 
>>   
>>   	return 0;
>>   }
>> -- 
>> 2.25.0