linux-kernel - Re: [PATCH 2/3] sgi-gru: Remove CONFIG_HUGETLB

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Date:   Sun, 21 Jul 2019 21:20:38 -0600
From:   William Kucharski <william.kucharski@...cle.com>
To:     Bharath Vedartham <linux.bhar@...il.com>
Cc:     arnd@...db.de, sivanich@....com, gregkh@...uxfoundation.org,
        ira.weiny@...el.com, jhubbard@...dia.com, jglisse@...hat.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/3] sgi-gru: Remove CONFIG_HUGETLB_PAGE ifdef

I suspect I'm being massively pedantic here, but the comments for atomic_pte_lookup() note:

 * Only supports Intel large pages (2MB only) on x86_64.
 *	ZZZ - hugepage support is incomplete

That makes me wonder how many systems using this hardware are actually configured with CONFIG_HUGETLB_PAGE.

I ask as in the most common case, this is likely introducing a few extra instructions and possibly an additional branch to a routine that is called per-fault.

So the nit-picky questions are:

1) Does the code really need to be cleaned up in this way?

2) If it does, does it make more sense (given the way pmd_large() is handled now in atomic_pte_lookup()) for this to be coded as:

if (unlikely(is_vm_hugetlb_page(vma)))
	*pageshift = HPAGE_SHIFT;
else
	*pageshift = PAGE_SHIFT;

In all likelihood, these questions are no-ops, and the optimizer may even make my questions completely moot, but I thought I might as well ask anyway.


> On Jul 21, 2019, at 9:58 AM, Bharath Vedartham <linux.bhar@...il.com> wrote:
> 
> is_vm_hugetlb_page has checks for whether CONFIG_HUGETLB_PAGE is defined
> or not. If CONFIG_HUGETLB_PAGE is not defined is_vm_hugetlb_page will
> always return false. There is no need to have an uneccessary
> CONFIG_HUGETLB_PAGE check in the code.
> 
> Cc: Ira Weiny <ira.weiny@...el.com>
> Cc: John Hubbard <jhubbard@...dia.com>
> Cc: Jérôme Glisse <jglisse@...hat.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Dimitri Sivanich <sivanich@....com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-mm@...ck.org
> Signed-off-by: Bharath Vedartham <linux.bhar@...il.com>
> ---
> drivers/misc/sgi-gru/grufault.c | 11 +++--------
> 1 file changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/misc/sgi-gru/grufault.c b/drivers/misc/sgi-gru/grufault.c
> index 61b3447..75108d2 100644
> --- a/drivers/misc/sgi-gru/grufault.c
> +++ b/drivers/misc/sgi-gru/grufault.c
> @@ -180,11 +180,8 @@ static int non_atomic_pte_lookup(struct vm_area_struct *vma,
> {
> 	struct page *page;
> 
> -#ifdef CONFIG_HUGETLB_PAGE
> 	*pageshift = is_vm_hugetlb_page(vma) ? HPAGE_SHIFT : PAGE_SHIFT;
> -#else
> -	*pageshift = PAGE_SHIFT;
> -#endif
> +
> 	if (get_user_pages(vaddr, 1, write ? FOLL_WRITE : 0, &page, NULL) <= 0)
> 		return -EFAULT;
> 	*paddr = page_to_phys(page);
> @@ -238,11 +235,9 @@ static int atomic_pte_lookup(struct vm_area_struct *vma, unsigned long vaddr,
> 		return 1;
> 
> 	*paddr = pte_pfn(pte) << PAGE_SHIFT;
> -#ifdef CONFIG_HUGETLB_PAGE
> +
> 	*pageshift = is_vm_hugetlb_page(vma) ? HPAGE_SHIFT : PAGE_SHIFT;
> -#else
> -	*pageshift = PAGE_SHIFT;
> -#endif
> +
> 	return 0;
> 
> err:
> -- 
> 2.7.4
>