RE: [PATCH v4] hugetlbfs: support split page table lock

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

Naoya Horiguchi wrote:
> Hi,
> 
> Kirill posted split_ptl patchset for thp today, so in this version
> I post only hugetlbfs part. I added Kconfig variables in following
> Kirill's patches (although without CONFIG_SPLIT_*_PTLOCK_CPUS.)
> 
> This patch changes many lines, but all are in hugetlbfs specific code,
> so I think we can apply this independent of thp patches.
> -----
> From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Date: Fri, 13 Sep 2013 18:12:30 -0400
> Subject: [PATCH v4] hugetlbfs: support split page table lock
> 
> Currently all of page table handling by hugetlbfs code are done under
> mm->page_table_lock. So when a process have many threads and they heavily
> access to the memory, lock contention happens and impacts the performance.
> 
> This patch makes hugepage support split page table lock so that we use
> page->ptl of the leaf node of page table tree which is pte for normal pages
> but can be pmd and/or pud for hugepages of some architectures.
> 
> ChangeLog v4:
>  - introduce arch dependent macro ARCH_ENABLE_SPLIT_HUGETLB_PTLOCK
>    (only defined for x86 for now)
>  - rename USE_SPLIT_PTLOCKS_HUGETLB to USE_SPLIT_HUGETLB_PTLOCKS
> 
> ChangeLog v3:
>  - disable split ptl for ppc with USE_SPLIT_PTLOCKS_HUGETLB.
>  - remove replacement in some architecture dependent code. This is justified
>    because an allocation of pgd/pud/pmd/pte entry can race with other
>    allocation, not with read/write access, so we can use different locks.
>    http://thread.gmane.org/gmane.linux.kernel.mm/106292/focus=106458
> 
> ChangeLog v2:
>  - add split ptl on other archs missed in v1
>  - drop changes on arch/{powerpc,tile}/mm/hugetlbpage.c
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> ---
>  arch/x86/Kconfig         |  4 +++
>  include/linux/hugetlb.h  | 20 +++++++++++
>  include/linux/mm_types.h |  2 ++
>  mm/Kconfig               |  3 ++
>  mm/hugetlb.c             | 92 +++++++++++++++++++++++++++++-------------------
>  mm/mempolicy.c           |  5 +--
>  mm/migrate.c             |  4 +--
>  mm/rmap.c                |  2 +-
>  8 files changed, 91 insertions(+), 41 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 6a5cf6a..5b83d14 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1884,6 +1884,10 @@ config ARCH_ENABLE_SPLIT_PMD_PTLOCK
>  	def_bool y
>  	depends on X86_64 || X86_PAE
>  
> +config ARCH_ENABLE_SPLIT_HUGETLB_PTLOCK
> +	def_bool y
> +	depends on X86_64 || X86_PAE
> +
>  menu "Power management and ACPI options"
>  
>  config ARCH_HIBERNATION_HEADER
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 0393270..2cdac68 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -80,6 +80,24 @@ extern const unsigned long hugetlb_zero, hugetlb_infinity;
>  extern int sysctl_hugetlb_shm_group;
>  extern struct list_head huge_boot_pages;
>  
> +#if USE_SPLIT_HUGETLB_PTLOCKS
> +#define huge_pte_lockptr(mm, ptep) ({__pte_lockptr(virt_to_page(ptep)); })
> +#else	/* !USE_SPLIT_HUGETLB_PTLOCKS */
> +#define huge_pte_lockptr(mm, ptep) ({&(mm)->page_table_lock; })
> +#endif	/* USE_SPLIT_HUGETLB_PTLOCKS */
> +
> +#define huge_pte_offset_lock(mm, address, ptlp)		\
> +({							\
> +	pte_t *__pte = huge_pte_offset(mm, address);	\
> +	spinlock_t *__ptl = NULL;			\
> +	if (__pte) {					\
> +		__ptl = huge_pte_lockptr(mm, __pte);	\
> +		*(ptlp) = __ptl;			\
> +		spin_lock(__ptl);			\
> +	}						\
> +	__pte;						\
> +})
> +

[ Disclaimer: I don't know much about hugetlb. ]

I don't think it's correct. Few points:

 - Hugetlb supports multiple page sizes: on x86_64 2M (PMD) and 1G (PUD).
   My patchset only implements it for PMD. We don't even initialize
   spinlock in struct page for PUD.
 - If we enable split PMD lock we should use it *globally*. With you patch
   we can end up with different locks used by hugetlb and rest of kernel
   to protect the same PMD table if USE_SPLIT_HUGETLB_PTLOCKS !=
   USE_SPLIT_PMD_PTLOCKS. It's just broken.

What we should really do is take split pmd lock (using pmd_lock*) if we
try to protect PMD level and fallback to mm->page_table_lock if we want to
protect upper levels.

>  /* arch callbacks */
>  
>  pte_t *huge_pte_alloc(struct mm_struct *mm,
> @@ -164,6 +182,8 @@ static inline void __unmap_hugepage_range(struct mmu_gather *tlb,
>  	BUG();
>  }
>  
> +#define huge_pte_lockptr(mm, ptep) 0
> +

NULL?

>  #endif /* !CONFIG_HUGETLB_PAGE */
>  
>  #define HUGETLB_ANON_FILE "anon_hugepage"

-- 
 Kirill A. Shutemov

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