linux-kernel - Re: [PATCH 02/37] mm, frame_vector: do not use get_user_pages

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Message-ID: <20151118122938.GB6097@quack.suse.cz>
Date:	Wed, 18 Nov 2015 13:29:38 +0100
From:	Jan Kara <jack@...e.cz>
To:	Dave Hansen <dave@...1.net>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	dave.hansen@...ux.intel.com, jack@...e.cz,
	akpm@...ux-foundation.org, aarcange@...hat.com
Subject: Re: [PATCH 02/37] mm, frame_vector: do not use
 get_user_pages_locked()

On Mon 16-11-15 19:35:14, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> get_user_pages_locked() appears to be for use when a caller needs
> to know that its lock on mmap_sem was invalidated by the gup
> call.
> 
> But, get_vaddr_frames() is not one of those users.  It
> unconditionally locks the mmap_sem and unconditionally unlocks it
> after the gup call.  It takes no special action and does not need
> to know whether its lock was invalidated or not.
> 
> Replace get_user_pages_locked() with a vanilla get_user_pages()
> and save a few lines of code.
> 
> Note that this was the *ONLY* use of get_user_pages_locked() in
> the entire kernel tree.

I've used get_user_pages_locked() because of a comment before that function
saying:

 * We can leverage the VM_FAULT_RETRY functionality in the page fault
 * paths better by using either get_user_pages_locked() or
 * get_user_pages_unlocked().
 *
 * get_user_pages_locked() is suitable to replace the form:
 *
 *      down_read(&mm->mmap_sem);
 *      do_something()
 *      get_user_pages(tsk, mm, ..., pages, NULL);
 *      up_read(&mm->mmap_sem);
 *
 *  to:
 *
 *      int locked = 1;
 *      down_read(&mm->mmap_sem);
 *      do_something()
 *      get_user_pages_locked(tsk, mm, ..., pages, &locked);
 *      if (locked)
 *          up_read(&mm->mmap_sem);

So I understood it as a way to reduce mmap_sem hold time by doing a try
first. Did I understand that comment wrong?

								Honza

> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> ---
> 
>  b/mm/frame_vector.c |    9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> diff -puN mm/frame_vector.c~remove-get_user_pages_locked-user mm/frame_vector.c
> --- a/mm/frame_vector.c~remove-get_user_pages_locked-user	2015-11-16 12:35:35.282169293 -0800
> +++ b/mm/frame_vector.c	2015-11-16 12:35:35.285169429 -0800
> @@ -40,7 +40,6 @@ int get_vaddr_frames(unsigned long start
>  	struct vm_area_struct *vma;
>  	int ret = 0;
>  	int err;
> -	int locked;
>  
>  	if (nr_frames == 0)
>  		return 0;
> @@ -49,7 +48,6 @@ int get_vaddr_frames(unsigned long start
>  		nr_frames = vec->nr_allocated;
>  
>  	down_read(&mm->mmap_sem);
> -	locked = 1;
>  	vma = find_vma_intersection(mm, start, start + 1);
>  	if (!vma) {
>  		ret = -EFAULT;
> @@ -58,8 +56,8 @@ int get_vaddr_frames(unsigned long start
>  	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP))) {
>  		vec->got_ref = true;
>  		vec->is_pfns = false;
> -		ret = get_user_pages_locked(current, mm, start, nr_frames,
> -			write, force, (struct page **)(vec->ptrs), &locked);
> +		ret = get_user_pages(current, mm, start, nr_frames,
> +			write, force, (struct page **)(vec->ptrs), NULL);
>  		goto out;
>  	}
>  
> @@ -87,8 +85,7 @@ int get_vaddr_frames(unsigned long start
>  		vma = find_vma_intersection(mm, start, start + 1);
>  	} while (vma && vma->vm_flags & (VM_IO | VM_PFNMAP));
>  out:
> -	if (locked)
> -		up_read(&mm->mmap_sem);
> +	up_read(&mm->mmap_sem);
>  	if (!ret)
>  		ret = -EFAULT;
>  	if (ret > 0)
> _
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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