Re: [PATCH v1 1/5] mm: pagewalk: Fix walk for hugepage tables

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 16/04/2021 à 00:43, Daniel Axtens a écrit :
> Hi Christophe,
> 
>> Pagewalk ignores hugepd entries and walk down the tables
>> as if it was traditionnal entries, leading to crazy result.
>>
>> Add walk_hugepd_range() and use it to walk hugepage tables.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>> ---
>>   mm/pagewalk.c | 54 +++++++++++++++++++++++++++++++++++++++++++++------
>>   1 file changed, 48 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
>> index e81640d9f177..410a9d8f7572 100644
>> --- a/mm/pagewalk.c
>> +++ b/mm/pagewalk.c
>> @@ -58,6 +58,32 @@ static int walk_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>>   	return err;
>>   }
>>   
>> +static int walk_hugepd_range(hugepd_t *phpd, unsigned long addr,
>> +			     unsigned long end, struct mm_walk *walk, int pdshift)
>> +{
>> +	int err = 0;
>> +#ifdef CONFIG_ARCH_HAS_HUGEPD
>> +	const struct mm_walk_ops *ops = walk->ops;
>> +	int shift = hugepd_shift(*phpd);
>> +	int page_size = 1 << shift;
>> +
>> +	if (addr & (page_size - 1))
>> +		return 0;
>> +
>> +	for (;;) {
>> +		pte_t *pte = hugepte_offset(*phpd, addr, pdshift);
>> +
>> +		err = ops->pte_entry(pte, addr, addr + page_size, walk);
>> +		if (err)
>> +			break;
>> +		if (addr >= end - page_size)
>> +			break;
>> +		addr += page_size;
>> +	}
> 
> Initially I thought this was a somewhat unintuitive way to structure
> this loop, but I see it parallels the structure of walk_pte_range_inner,
> so I think the consistency is worth it.
> 
> I notice the pte walking code potentially takes some locks: does this
> code need to do that?
> 
> arch/powerpc/mm/hugetlbpage.c says that hugepds are protected by the
> mm->page_table_lock, but I don't think we're taking it in this code.

I'll add it, thanks.

> 
>> +#endif
>> +	return err;
>> +}
>> +
>>   static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>>   			  struct mm_walk *walk)
>>   {
>> @@ -108,7 +134,10 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>>   				goto again;
>>   		}
>>   
>> -		err = walk_pte_range(pmd, addr, next, walk);
>> +		if (is_hugepd(__hugepd(pmd_val(*pmd))))
>> +			err = walk_hugepd_range((hugepd_t *)pmd, addr, next, walk, PMD_SHIFT);
>> +		else
>> +			err = walk_pte_range(pmd, addr, next, walk);
>>   		if (err)
>>   			break;
>>   	} while (pmd++, addr = next, addr != end);
>> @@ -157,7 +186,10 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
>>   		if (pud_none(*pud))
>>   			goto again;
>>   
>> -		err = walk_pmd_range(pud, addr, next, walk);
>> +		if (is_hugepd(__hugepd(pud_val(*pud))))
>> +			err = walk_hugepd_range((hugepd_t *)pud, addr, next, walk, PUD_SHIFT);
>> +		else
>> +			err = walk_pmd_range(pud, addr, next, walk);
> 
> I'm a bit worried you might end up calling into walk_hugepd_range with
> ops->pte_entry == NULL, and then jumping to 0.

You are right, I missed it.
I'll bail out of walk_hugepd_range() when ops->pte_entry is NULL.


> 
> static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
> 			  struct mm_walk *walk)
> {
> ...
>          pud = pud_offset(p4d, addr);
> 	do {
>                  ...
>                  if ((!walk->vma && (pud_leaf(*pud) || !pud_present(*pud))) ||
> 		    walk->action == ACTION_CONTINUE ||
> 		    !(ops->pmd_entry || ops->pte_entry)) <<< THIS CHECK
> 			continue;
>                  ...
> 		if (is_hugepd(__hugepd(pud_val(*pud))))
> 			err = walk_hugepd_range((hugepd_t *)pud, addr, next, walk, PUD_SHIFT);
> 		else
> 			err = walk_pmd_range(pud, addr, next, walk);
> 		if (err)
> 			break;
> 	} while (pud++, addr = next, addr != end);
> 
> walk_pud_range will proceed if there is _either_ an ops->pmd_entry _or_
> an ops->pte_entry, but walk_hugepd_range will call ops->pte_entry
> unconditionally.
> 
> The same issue applies to walk_{p4d,pgd}_range...
> 
> Kind regards,
> Daniel
> 

Thanks
Christophe

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