linux-kernel - Re: [RFC PATCH 1/2] mm/hugetlbfs: Attempt PUD_SIZE mapping alignment if PMD sharing enabled

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Date:	Tue, 29 Mar 2016 11:50:47 +0800
From:	"Hillf Danton" <hillf.zj@...baba-inc.com>
To:	"'Mike Kravetz'" <mike.kravetz@...cle.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, <x86@...nel.org>
Cc:	"'Hugh Dickins'" <hughd@...gle.com>,
	"'Naoya Horiguchi'" <n-horiguchi@...jp.nec.com>,
	"'Kirill A. Shutemov'" <kirill.shutemov@...ux.intel.com>,
	"'David Rientjes'" <rientjes@...gle.com>,
	"'Dave Hansen'" <dave.hansen@...ux.intel.com>,
	"'Thomas Gleixner'" <tglx@...utronix.de>,
	"'Ingo Molnar'" <mingo@...hat.com>,
	"'H. Peter Anvin'" <hpa@...or.com>,
	"'Catalin Marinas'" <catalin.marinas@....com>,
	"'Will Deacon'" <will.deacon@....com>,
	"'Steve Capper'" <steve.capper@...aro.org>,
	"'Andrew Morton'" <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 1/2] mm/hugetlbfs: Attempt PUD_SIZE mapping alignment if PMD sharing enabled

> 
> When creating a hugetlb mapping, attempt PUD_SIZE alignment if the
> following conditions are met:
> - Address passed to mmap or shmat is NULL
> - The mapping is flaged as shared
> - The mapping is at least PUD_SIZE in length
> If a PUD_SIZE aligned mapping can not be created, then fall back to a
> huge page size mapping.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> ---
>  fs/hugetlbfs/inode.c | 29 +++++++++++++++++++++++++++--
>  1 file changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index 540ddc9..22b2e38 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -175,6 +175,17 @@ hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
>  	struct vm_area_struct *vma;
>  	struct hstate *h = hstate_file(file);
>  	struct vm_unmapped_area_info info;
> +	bool pud_size_align = false;
> +	unsigned long ret_addr;
> +
> +	/*
> +	 * If PMD sharing is enabled, align to PUD_SIZE to facilitate
> +	 * sharing.  Only attempt alignment if no address was passed in,
> +	 * flags indicate sharing and size is big enough.
> +	 */
> +	if (IS_ENABLED(CONFIG_ARCH_WANT_HUGE_PMD_SHARE) &&
> +	    !addr && flags & MAP_SHARED && len >= PUD_SIZE)
> +		pud_size_align = true;
> 
>  	if (len & ~huge_page_mask(h))
>  		return -EINVAL;
> @@ -199,9 +210,23 @@ hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
>  	info.length = len;
>  	info.low_limit = TASK_UNMAPPED_BASE;
>  	info.high_limit = TASK_SIZE;
> -	info.align_mask = PAGE_MASK & ~huge_page_mask(h);
> +	if (pud_size_align)
> +		info.align_mask = PAGE_MASK & (PUD_SIZE - 1);
> +	else
> +		info.align_mask = PAGE_MASK & ~huge_page_mask(h);
>  	info.align_offset = 0;
> -	return vm_unmapped_area(&info);
> +	ret_addr = vm_unmapped_area(&info);
> +
> +	/*
> +	 * If failed with PUD_SIZE alignment, try again with huge page
> +	 * size alignment.
> +	 */

Can we avoid going another round as long as it is a file with
the PUD page size?

Hillf
> +	if ((ret_addr & ~PAGE_MASK) && pud_size_align) {
> +		info.align_mask = PAGE_MASK & ~huge_page_mask(h);
> +		ret_addr = vm_unmapped_area(&info);
> +	}
> +
> +	return ret_addr;
>  }
>  #endif
> 
> --
> 2.4.3