linux-kernel - Re: [PATCH] mm: unexport __get_user

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Date:   Mon, 24 Oct 2016 13:15:23 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>, Jan Kara <jack@...e.cz>,
        Hugh Dickins <hughd@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Rik van Riel <riel@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: unexport __get_user_pages()



On 24/10/2016 11:57, Lorenzo Stoakes wrote:
> This patch unexports the low-level __get_user_pages() function. Recent
> refactoring of the get_user_pages* functions allow flags to be passed through
> get_user_pages() which eliminates the need for access to this function from its
> one user, kvm.
> 
> We can see that the 2 calls to get_user_pages() which replace __get_user_pages()
> in kvm_main.c are equivalent by examining their call stacks:
> 
> get_user_page_nowait():
>   get_user_pages(start, 1, flags, page, NULL)
>   __get_user_pages_locked(current, current->mm, start, 1, page, NULL, NULL,
> 			  false, flags | FOLL_TOUCH)
>   __get_user_pages(current, current->mm, start, 1,
> 		   flags | FOLL_TOUCH | FOLL_GET, page, NULL, NULL)
> 
> check_user_page_hwpoison():
>   get_user_pages(addr, 1, flags, NULL, NULL)
>   __get_user_pages_locked(current, current->mm, addr, 1, NULL, NULL, NULL,
> 			  false, flags | FOLL_TOUCH)
>   __get_user_pages(current, current->mm, addr, 1, flags | FOLL_TOUCH, NULL,
> 		   NULL, NULL)
> 
> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> ---
>  include/linux/mm.h  |  4 ----
>  mm/gup.c            |  3 +--
>  mm/nommu.c          |  2 +-
>  virt/kvm/kvm_main.c | 10 ++++------
>  4 files changed, 6 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 3a19185..a92c8d7 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1271,10 +1271,6 @@ extern int access_process_vm(struct task_struct *tsk, unsigned long addr, void *
>  extern int access_remote_vm(struct mm_struct *mm, unsigned long addr,
>  		void *buf, int len, unsigned int gup_flags);
>  
> -long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
> -		      unsigned long start, unsigned long nr_pages,
> -		      unsigned int foll_flags, struct page **pages,
> -		      struct vm_area_struct **vmas, int *nonblocking);
>  long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
>  			    unsigned long start, unsigned long nr_pages,
>  			    unsigned int gup_flags, struct page **pages,
> diff --git a/mm/gup.c b/mm/gup.c
> index 7aa113c..ec4f827 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -526,7 +526,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
>   * instead of __get_user_pages. __get_user_pages should be used only if
>   * you need some special @gup_flags.
>   */
> -long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
> +static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>  		unsigned long start, unsigned long nr_pages,
>  		unsigned int gup_flags, struct page **pages,
>  		struct vm_area_struct **vmas, int *nonblocking)
> @@ -631,7 +631,6 @@ long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>  	} while (nr_pages);
>  	return i;
>  }
> -EXPORT_SYMBOL(__get_user_pages);
>  
>  bool vma_permits_fault(struct vm_area_struct *vma, unsigned int fault_flags)
>  {
> diff --git a/mm/nommu.c b/mm/nommu.c
> index db5fd17..8b8faaf 100644
> --- a/mm/nommu.c
> +++ b/mm/nommu.c
> @@ -109,7 +109,7 @@ unsigned int kobjsize(const void *objp)
>  	return PAGE_SIZE << compound_order(page);
>  }
>  
> -long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
> +static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>  		      unsigned long start, unsigned long nr_pages,
>  		      unsigned int foll_flags, struct page **pages,
>  		      struct vm_area_struct **vmas, int *nonblocking)
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 28510e7..2907b7b 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1346,21 +1346,19 @@ unsigned long kvm_vcpu_gfn_to_hva_prot(struct kvm_vcpu *vcpu, gfn_t gfn, bool *w
>  static int get_user_page_nowait(unsigned long start, int write,
>  		struct page **page)
>  {
> -	int flags = FOLL_TOUCH | FOLL_NOWAIT | FOLL_HWPOISON | FOLL_GET;
> +	int flags = FOLL_NOWAIT | FOLL_HWPOISON;
>  
>  	if (write)
>  		flags |= FOLL_WRITE;
>  
> -	return __get_user_pages(current, current->mm, start, 1, flags, page,
> -			NULL, NULL);
> +	return get_user_pages(start, 1, flags, page, NULL);
>  }
>  
>  static inline int check_user_page_hwpoison(unsigned long addr)
>  {
> -	int rc, flags = FOLL_TOUCH | FOLL_HWPOISON | FOLL_WRITE;
> +	int rc, flags = FOLL_HWPOISON | FOLL_WRITE;
>  
> -	rc = __get_user_pages(current, current->mm, addr, 1,
> -			      flags, NULL, NULL, NULL);
> +	rc = get_user_pages(addr, 1, flags, NULL, NULL);
>  	return rc == -EHWPOISON;
>  }
>  
> 

Acked-by: Paolo Bonzini <pbonzini@...hat.com>