Re: Kill off hugepage_vma()

["'David Gibson'" <david@gibson.dropbear.id.au>]

On Fri, Apr 16, 2004 at 03:14:39PM -0700, Chen, Kenneth W wrote:
> Original post on LKML:
> http://www.ussg.iu.edu/hypermail/linux/kernel/0404.2/0017.html
> 
> >>>> David Gibson wrote on Thursday, April 15, 2004 11:22 PM
> > Kenneth, does the patch below look reasonable?  It's close to trivial
> > on everything except IA64, which I can't easily test on.  Actually, it
> > occurs to me that this may even fix a bug on IA64 - if follow_page()
> > was called on an address in region 1 that wasn't in a VMA, then I
> > think it would attempt to follow it as a normal page, which sounds
> > bad.
> 
> Thanks to well written code in the caller of follow_page(), that they
> always call find_extend_vma() to make sure address belongs to a vma
> before calling down to follow_page().  So there is no bug here.

Ah, good point.

> Minor FYI: on ia64, huge page address is in region 4.

Oops.

> > hugepage_vma() is both misleadingly named and unnecessary.  On most
> > archs it always returns NULL, and on IA64 the vma it returns is never
> > used.  The function's real purpose is to determine whether the address
> > it is passed is a special hugepage address which must be looked up in
> > hugepage pagetables, rather than being looked up in the normal
> > pagetables (which might have specially marked hugepage PMDs or PTEs).
> 
> One way or the other, the check has to be there.  If the name is vague,
> a new name can be proposed.
> 
> > This patch kills off hugepage_vma() and folds the logic it really
> > needs into follow_huge_addr().  That now returns a (page *) if called
> > on a special hugepage address, and an error encoded with ERR_PTR
> > otherwise.  This also requires tweaking the IA64 code to check that
> > the hugepage PTE is present in follow_huge_addr() - previously this
> > was guaranteed, since it was only called if the address was in an
> > existing hugepage VMA, and hugepages are always prefaulted.
> 
> Why not do one step further? Instead of call a dummy function that
> always return false, why not stub it out in the header file for arch
> that don't need follow_huge_addr? This would speedup everyone.

I didn't do that purely to avoid proliferating more ARCH_HAS_BLAH
defines (we already have 2 for hugepage).  It has no more overhead
than the current version of course.  I have no strong preference on
this point.

However, I folded the check logic into hugetlb_follow_addr() rather
than leaving it as two functions for a reason - akpm indicated that he
thought the former was preferable.  I like that approach slightly
better myself, too, although it's not a strong preference.

If it were to be two functions though, I really dislike the name
prepare_follow_huge_addr().  I'd prefer, say, in_hugepage_region().

> I propose the following patch:
> 
> 
> diff -Nur linux-2.6.5/arch/i386/mm/hugetlbpage.c linux-2.6.5.htlb/arch/i386/mm/hugetlbpage.c
> +++ linux-2.6.5.htlb/arch/i386/mm/hugetlbpage.c	2004-04-16 14:15:28.000000000 -0700
> @@ -182,18 +182,6 @@
> 
>  #else
> 
> -struct page *
> -follow_huge_addr(struct mm_struct *mm,
> -	struct vm_area_struct *vma, unsigned long address, int write)
> -{
> -	return NULL;
> -}
> -
> -struct vm_area_struct *hugepage_vma(struct mm_struct *mm, unsigned long addr)
> -{
> -	return NULL;
> -}
> -
>  int pmd_huge(pmd_t pmd)
>  {
>  	return !!(pmd_val(pmd) & _PAGE_PSE);
> diff -Nur linux-2.6.5/arch/ia64/mm/hugetlbpage.c linux-2.6.5.htlb/arch/ia64/mm/hugetlbpage.c
> +++ linux-2.6.5.htlb/arch/ia64/mm/hugetlbpage.c	2004-04-16 14:20:35.000000000 -0700
> @@ -149,19 +149,7 @@
>  	return i;
>  }
> 
> -struct vm_area_struct *hugepage_vma(struct mm_struct *mm, unsigned long addr)
> -{
> -	if (mm->used_hugetlb) {
> -		if (REGION_NUMBER(addr) = REGION_HPAGE) {
> -			struct vm_area_struct *vma = find_vma(mm, addr);
> -			if (vma && is_vm_hugetlb_page(vma))
> -				return vma;
> -		}
> -	}
> -	return NULL;
> -}
> -
> -struct page *follow_huge_addr(struct mm_struct *mm, struct vm_area_struct *vma, unsigned long addr, int write)
> +struct page *follow_huge_addr(struct mm_struct *mm, unsigned long addr, int write)
>  {
>  	struct page *page;
>  	pte_t *ptep;
> diff -Nur linux-2.6.5/arch/ppc64/mm/hugetlbpage.c linux-2.6.5.htlb/arch/ppc64/mm/hugetlbpage.c
> +++ linux-2.6.5.htlb/arch/ppc64/mm/hugetlbpage.c	2004-04-16 14:37:19.000000000 -0700
> @@ -334,18 +334,6 @@
>  	return i;
>  }
> 
> -struct page *
> -follow_huge_addr(struct mm_struct *mm,
> -	struct vm_area_struct *vma, unsigned long address, int write)
> -{
> -	return NULL;
> -}
> -
> -struct vm_area_struct *hugepage_vma(struct mm_struct *mm, unsigned long addr)
> -{
> -	return NULL;
> -}
> -
>  int pmd_huge(pmd_t pmd)
>  {
>  	return pmd_hugepage(pmd);
> diff -Nur linux-2.6.5/arch/sh/mm/hugetlbpage.c linux-2.6.5.htlb/arch/sh/mm/hugetlbpage.c
> +++ linux-2.6.5.htlb/arch/sh/mm/hugetlbpage.c	2004-04-16 14:16:09.000000000 -0700
> @@ -165,18 +165,6 @@
>  	return i;
>  }
> 
> -struct page *follow_huge_addr(struct mm_struct *mm,
> -			      struct vm_area_struct *vma,
> -			      unsigned long address, int write)
> -{
> -	return NULL;
> -}
> -
> -struct vm_area_struct *hugepage_vma(struct mm_struct *mm, unsigned long addr)
> -{
> -	return NULL;
> -}
> -
>  int pmd_huge(pmd_t pmd)
>  {
>  	return 0;
> diff -Nur linux-2.6.5/arch/sparc64/mm/hugetlbpage.c linux-2.6.5.htlb/arch/sparc64/mm/hugetlbpage.c
> +++ linux-2.6.5.htlb/arch/sparc64/mm/hugetlbpage.c	2004-04-16 14:16:24.000000000 -0700
> @@ -162,18 +162,6 @@
>  	return i;
>  }
> 
> -struct page *follow_huge_addr(struct mm_struct *mm,
> -			      struct vm_area_struct *vma,
> -			      unsigned long address, int write)
> -{
> -	return NULL;
> -}
> -
> -struct vm_area_struct *hugepage_vma(struct mm_struct *mm, unsigned long addr)
> -{
> -	return NULL;
> -}
> -
>  int pmd_huge(pmd_t pmd)
>  {
>  	return 0;
> diff -Nur linux-2.6.5/include/asm-ia64/page.h linux-2.6.5.htlb/include/asm-ia64/page.h
> +++ linux-2.6.5.htlb/include/asm-ia64/page.h	2004-04-16 14:07:52.000000000 -0700
> @@ -47,6 +47,7 @@
> 
>  # define HAVE_ARCH_HUGETLB_UNMAPPED_AREA
>  # define ARCH_HAS_HUGEPAGE_ONLY_RANGE
> +# define ARCH_HAS_FOLLOW_HUGE_ADDR
>  #endif /* CONFIG_HUGETLB_PAGE */
> 
>  #ifdef __ASSEMBLY__
> @@ -123,6 +124,7 @@
>  # define is_hugepage_only_range(addr, len)		\
>  	 (REGION_NUMBER(addr) = REGION_HPAGE &&	\
>  	  REGION_NUMBER((addr)+(len)) = REGION_HPAGE)
> +# define prepare_follow_huge_addr(addr)	(REGION_NUMBER(addr) = REGION_HPAGE)
>  extern unsigned int hpage_shift;
>  #endif
> 
> diff -Nur linux-2.6.5/include/linux/hugetlb.h linux-2.6.5.htlb/include/linux/hugetlb.h
> +++ linux-2.6.5.htlb/include/linux/hugetlb.h	2004-04-16 14:14:16.000000000 -0700
> @@ -22,10 +22,6 @@
>  int hugetlb_report_meminfo(char *);
>  int is_hugepage_mem_enough(size_t);
>  unsigned long hugetlb_total_pages(void);
> -struct page *follow_huge_addr(struct mm_struct *mm, struct vm_area_struct *vma,
> -			unsigned long address, int write);
> -struct vm_area_struct *hugepage_vma(struct mm_struct *mm,
> -					unsigned long address);
>  struct page *follow_huge_pmd(struct mm_struct *mm, unsigned long address,
>  				pmd_t *pmd, int write);
>  int is_aligned_hugepage_range(unsigned long addr, unsigned long len);
> @@ -55,6 +51,13 @@
>  int prepare_hugepage_range(unsigned long addr, unsigned long len);
>  #endif
> 
> +#ifndef ARCH_HAS_FOLLOW_HUGE_ADDR
> +#define prepare_follow_huge_addr(addr)		0
> +#define follow_huge_addr(mm, addr, write)	0
> +#else
> +struct page *follow_huge_addr(struct mm_struct *mm, unsigned long address, int write);
> +#endif
> +
>  #else /* !CONFIG_HUGETLB_PAGE */
> 
>  static inline int is_vm_hugetlb_page(struct vm_area_struct *vma)
> @@ -67,7 +70,8 @@
>  }
> 
>  #define follow_hugetlb_page(m,v,p,vs,a,b,i)	({ BUG(); 0; })
> -#define follow_huge_addr(mm, vma, addr, write)	0
> +#define prepare_follow_huge_addr(addr)		0
> +#define follow_huge_addr(mm, addr, write)	0
>  #define copy_hugetlb_page_range(src, dst, vma)	({ BUG(); 0; })
>  #define hugetlb_prefault(mapping, vma)		({ BUG(); 0; })
>  #define zap_hugepage_range(vma, start, len)	BUG()
> @@ -75,7 +79,6 @@
>  #define huge_page_release(page)			BUG()
>  #define is_hugepage_mem_enough(size)		0
>  #define hugetlb_report_meminfo(buf)		0
> -#define hugepage_vma(mm, addr)			0
>  #define mark_mm_hugetlb(mm, vma)		do { } while (0)
>  #define follow_huge_pmd(mm, addr, pmd, write)	0
>  #define is_aligned_hugepage_range(addr, len)	0
> diff -Nur linux-2.6.5/mm/memory.c linux-2.6.5.htlb/mm/memory.c
> +++ linux-2.6.5.htlb/mm/memory.c	2004-04-16 14:20:49.000000000 -0700
> @@ -629,11 +629,9 @@
>  	pmd_t *pmd;
>  	pte_t *ptep, pte;
>  	unsigned long pfn;
> -	struct vm_area_struct *vma;
> 
> -	vma = hugepage_vma(mm, address);
> -	if (vma)
> -		return follow_huge_addr(mm, vma, address, write);
> +	if (prepare_follow_huge_addr(address))
> +		return follow_huge_addr(mm, address, write);
> 
>  	pgd = pgd_offset(mm, address);
>  	if (pgd_none(*pgd) || pgd_bad(*pgd))
> 
> 

-- 
David Gibson			| For every complex problem there is a
david AT gibson.dropbear.id.au	| solution which is simple, neat and
				| wrong.
http://www.ozlabs.org/people/dgibson