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

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Date:   Mon, 22 Jul 2019 23:20:22 +0530
From:   Bharath Vedartham <linux.bhar@...il.com>
To:     William Kucharski <william.kucharski@...cle.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

On Sun, Jul 21, 2019 at 09:20:38PM -0600, William Kucharski wrote:
> 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.
> 
That sounds reasonable. I am not really sure as to how much of 
an improvement it would be, the condition will be evaluated eitherways
AFAIK? Eitherways, the ternary operator does not look good. I ll make a
version 2 of this.
> > 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
> > 
>