Re: [PATCH RFC 2/6] mm: PGTABLE_HAS_P[MU]D_LEAVES config options

[LEROY Christophe <christophe.leroy2@cs-soprasteria.com>]

Le 22/08/2024 à 21:16, Peter Xu a écrit :
> On Thu, Aug 22, 2024 at 05:22:03PM +0000, LEROY Christophe wrote:
>>
>>
>> Le 18/07/2024 à 00:02, Peter Xu a écrit :
>>> Introduce two more sub-options for PGTABLE_HAS_HUGE_LEAVES:
>>>
>>>     - PGTABLE_HAS_PMD_LEAVES: set when there can be PMD mappings
>>>     - PGTABLE_HAS_PUD_LEAVES: set when there can be PUD mappings
>>>
>>> It will help to identify whether the current build may only want PMD
>>> helpers but not PUD ones, as these sub-options will also check against the
>>> arch support over HAVE_ARCH_TRANSPARENT_HUGEPAGE[_PUD].
>>>
>>> Note that having them depend on HAVE_ARCH_TRANSPARENT_HUGEPAGE[_PUD] is
>>> still some intermediate step.  The best way is to have an option say
>>> "whether arch XXX supports PMD/PUD mappings" and so on.  However let's
>>> leave that for later as that's the easy part.  So far, we use these options
>>> to stably detect per-arch huge mapping support.
>>>
>>> Signed-off-by: Peter Xu <peterx@redhat.com>
>>> ---
>>>    include/linux/huge_mm.h | 10 +++++++---
>>>    mm/Kconfig              |  6 ++++++
>>>    2 files changed, 13 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>> index 711632df7edf..37482c8445d1 100644
>>> --- a/include/linux/huge_mm.h
>>> +++ b/include/linux/huge_mm.h
>>> @@ -96,14 +96,18 @@ extern struct kobj_attribute thpsize_shmem_enabled_attr;
>>>    #define thp_vma_allowable_order(vma, vm_flags, tva_flags, order) \
>>>    	(!!thp_vma_allowable_orders(vma, vm_flags, tva_flags, BIT(order)))
>>>    
>>> -#ifdef CONFIG_PGTABLE_HAS_HUGE_LEAVES
>>> -#define HPAGE_PMD_SHIFT PMD_SHIFT
>>> +#ifdef CONFIG_PGTABLE_HAS_PUD_LEAVES
>>>    #define HPAGE_PUD_SHIFT PUD_SHIFT
>>>    #else
>>> -#define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
>>>    #define HPAGE_PUD_SHIFT ({ BUILD_BUG(); 0; })
>>>    #endif
>>>    
>>> +#ifdef CONFIG_PGTABLE_HAS_PMD_LEAVES
>>> +#define HPAGE_PMD_SHIFT PMD_SHIFT
>>> +#else
>>> +#define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
>>> +#endif
>>> +
>>>    #define HPAGE_PMD_ORDER (HPAGE_PMD_SHIFT-PAGE_SHIFT)
>>>    #define HPAGE_PMD_NR (1<<HPAGE_PMD_ORDER)
>>>    #define HPAGE_PMD_MASK	(~(HPAGE_PMD_SIZE - 1))
>>> diff --git a/mm/Kconfig b/mm/Kconfig
>>> index 60796402850e..2dbdc088dee8 100644
>>> --- a/mm/Kconfig
>>> +++ b/mm/Kconfig
>>> @@ -860,6 +860,12 @@ endif # TRANSPARENT_HUGEPAGE
>>>    config PGTABLE_HAS_HUGE_LEAVES
>>>    	def_bool TRANSPARENT_HUGEPAGE || HUGETLB_PAGE
>>>    
>>> +config PGTABLE_HAS_PMD_LEAVES
>>> +	def_bool HAVE_ARCH_TRANSPARENT_HUGEPAGE && PGTABLE_HAS_HUGE_LEAVES
>>> +
>>> +config PGTABLE_HAS_PUD_LEAVES
>>> +	def_bool HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD && PGTABLE_HAS_HUGE_LEAVES
>>> +
>>
>> What if an architecture has hugepages at PMD and/or PUD level and
>> doesn't support THP ?
> 
> What's the arch to be discussed here?

It is LOONGARCH and MIPS, they provide pud_leaf() that can return true 
even when they have no PUD.

> 
> The whole purpose of this series so far is trying to make some pmd/pud
> helpers that only defined with CONFIG_THP=on to be available even if not.
> It means this series alone (or any future plan) shouldn't affect any arch
> that has CONFIG_THP=off always.
> 
> But logically I think we should need some config option just to say "this
> arch supports pmd mappings" indeed, even if CONFIG_THP=off.  When that's
> there, we should perhaps add that option into this equation so
> PGTABLE_HAS_*_LEAVES will also be selected in that case.
> 

Why is an option needed for that ? If pmd_leaf() returns always false, 
it means the arch doesn't support pmd mappings and if properly used all 
related code should fold away without a config option, shouldn't it ?