linux-kernel - Re: [RFC PATCH 2/2] x86/hugetlb: Attempt PUD_SIZE mapping alignment if PMD sharing enabled

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Date:	Tue, 29 Mar 2016 10:35:10 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Mike Kravetz <mike.kravetz@...cle.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org, x86@...nel.org,
	Hugh Dickins <hughd@...gle.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.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 2/2] x86/hugetlb: Attempt PUD_SIZE mapping alignment
 if PMD sharing enabled


* Mike Kravetz <mike.kravetz@...cle.com> 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.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> ---
>  arch/x86/mm/hugetlbpage.c | 64 ++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 61 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
> index 42982b2..4f53af5 100644
> --- a/arch/x86/mm/hugetlbpage.c
> +++ b/arch/x86/mm/hugetlbpage.c
> @@ -78,14 +78,39 @@ static unsigned long hugetlb_get_unmapped_area_bottomup(struct file *file,
>  {
>  	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;
>  
>  	info.flags = 0;
>  	info.length = len;
>  	info.low_limit = current->mm->mmap_legacy_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.
> +	 */
> +	if ((ret_addr & ~PAGE_MASK) && pud_size_align) {
> +		info.align_mask = PAGE_MASK & ~huge_page_mask(h);
> +		ret_addr = vm_unmapped_area(&info);
> +	}

So AFAICS 'ret_addr' is either page aligned, or is an error code. Wouldn't it be a 
lot easier to read to say:

	if ((long)ret_addr > 0 && pud_size_align) {
		info.align_mask = PAGE_MASK & ~huge_page_mask(h);
		ret_addr = vm_unmapped_area(&info);
	}

	return ret_addr;

to make it clear that it's about error handling, not some alignment 
requirement/restriction?

>  	/*
> +	 * If failed with PUD_SIZE alignment, try again with huge page
> +	 * size alignment.
> +	 */
> +	if ((addr & ~PAGE_MASK) && pud_size_align) {
> +		info.align_mask = PAGE_MASK & ~huge_page_mask(h);
> +		addr = vm_unmapped_area(&info);
> +	}

Ditto.

>  		addr = vm_unmapped_area(&info);
> +
> +		/*
> +		 * If failed again with PUD_SIZE alignment, finally try with
> +		 * huge page size alignment.
> +		 */
> +		if (addr & ~PAGE_MASK) {
> +			info.align_mask = PAGE_MASK & ~huge_page_mask(h);
> +			addr = vm_unmapped_area(&info);

Ditto.

Thanks,

	Ingo