linux-kernel - Re: [PATCH RFC 2/6] mm: PGTABLE_HAS_P[MU]D

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Message-ID: <dcdde9fc-7e7c-45a8-8dc7-7f7ed13b81ec@cs-soprasteria.com>
Date: Thu, 22 Aug 2024 17:22:03 +0000
From: LEROY Christophe <christophe.leroy2@...soprasteria.com>
To: Peter Xu <peterx@...hat.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>
CC: Vlastimil Babka <vbabka@...e.cz>, David Hildenbrand <david@...hat.com>,
	Oscar Salvador <osalvador@...e.de>, "linux-s390@...r.kernel.org"
	<linux-s390@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Wilcox <willy@...radead.org>, Dan Williams
	<dan.j.williams@...el.com>, Michal Hocko <mhocko@...nel.org>,
	"linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
	"sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>, Alex Williamson
	<alex.williamson@...hat.com>, Jason Gunthorpe <jgg@...dia.com>,
	"x86@...nel.org" <x86@...nel.org>, Alistair Popple <apopple@...dia.com>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, Ryan Roberts <ryan.roberts@....com>,
	Hugh Dickins <hughd@...gle.com>, Axel Rasmussen <axelrasmussen@...gle.com>
Subject: Re: [PATCH RFC 2/6] mm: PGTABLE_HAS_P[MU]D_LEAVES config options



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@...hat.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 ?

Christophe

>   #
>   # UP and nommu archs use km based percpu allocator
>   #