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

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Date:	Thu, 31 Mar 2016 02:18:51 +0000
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Mike Kravetz <mike.kravetz@...cle.com>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"Hugh Dickins" <hughd@...gle.com>,
	Hillf Danton <hillf.zj@...baba-inc.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

On Mon, Mar 28, 2016 at 06:12:49PM -0700, Mike Kravetz wrote:
> 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.

It would be kinder if the patch description includes why this change.
Simply "to facilitate pmd sharing" is helpful for someone who read
"git log".

> 
> 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;

This code will have duplicates in the next patch, so how about checking
this in a separate check routine?

Thanks,
Naoya Horiguchi