Re: [PATCH 1/2] vmsplice: unmap gifted pages for recipient

[Dave Hansen <dave@sr71.net>]

On 10/07/2013 01:21 PM, Robert C Jennings wrote:
>  		spd.partial[page_nr].offset = loff;
>  		spd.partial[page_nr].len = this_len;
> +		spd.partial[page_nr].useraddr = index << PAGE_CACHE_SHIFT;
>  		len -= this_len;
>  		loff = 0;
>  		spd.nr_pages++;
> @@ -656,6 +702,7 @@ ssize_t default_file_splice_read(struct file *in, loff_t *ppos,
>  		this_len = min_t(size_t, vec[i].iov_len, res);
>  		spd.partial[i].offset = 0;
>  		spd.partial[i].len = this_len;
> +		spd.partial[i].useraddr = (unsigned long)vec[i].iov_base;
>  		if (!this_len) {
>  			__free_page(spd.pages[i]);
>  			spd.pages[i] = NULL;
> @@ -1475,6 +1522,8 @@ static int get_iovec_page_array(const struct iovec __user *iov,
>  
>  			partial[buffers].offset = off;
>  			partial[buffers].len = plen;
> +			partial[buffers].useraddr = (unsigned long)base;
> +			base = (void*)((unsigned long)base + PAGE_SIZE);
>  
>  			off = 0;
>  			len -= plen;
> diff --git a/include/linux/splice.h b/include/linux/splice.h
> index 74575cb..56661e3 100644
> --- a/include/linux/splice.h
> +++ b/include/linux/splice.h
> @@ -44,6 +44,7 @@ struct partial_page {
>  	unsigned int offset;
>  	unsigned int len;
>  	unsigned long private;
> +	unsigned long useraddr;
>  };

"useraddr" confuses me.  You make it an 'unsigned long', yet two of the
three assignments are from "void __user *".  The other assignment:

	spd.partial[page_nr].useraddr = index << PAGE_CACHE_SHIFT;

'index' looks to be the offset inside the file, not a user address, so
I'm confused what that is doing.

Could you elaborate a little more on why 'useraddr' is suddenly needed
in these patches?  How is "index << PAGE_CACHE_SHIFT" a virtual address?
 Also, are we losing any of the advantages of sparse checking since
'useraddr' is without the __user annotation?

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