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

[Vlastimil Babka <vbabka@suse.cz>]

On 10/07/2013 10:21 PM, Robert C Jennings wrote:
> Introduce use of the unused SPLICE_F_MOVE flag for vmsplice to zap
> pages.
> 
> When vmsplice is called with flags (SPLICE_F_GIFT | SPLICE_F_MOVE) the
> writer's gift'ed pages would be zapped.  This patch supports further work
> to move vmsplice'd pages rather than copying them.  That patch has the
> restriction that the page must not be mapped by the source for the move,
> otherwise it will fall back to copying the page.
> 
> Signed-off-by: Matt Helsley <matt.helsley@gmail.com>
> Signed-off-by: Robert C Jennings <rcj@linux.vnet.ibm.com>
> ---
> Since the RFC went out I have coalesced the zap_page_range() call to
> operate on VMAs rather than calling this for each page.  For a 256MB
> vmsplice this reduced the write side 50% from the RFC.
> ---
>  fs/splice.c            | 51 +++++++++++++++++++++++++++++++++++++++++++++++++-
>  include/linux/splice.h |  1 +
>  2 files changed, 51 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/splice.c b/fs/splice.c
> index 3b7ee65..a62d61e 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -188,12 +188,17 @@ ssize_t splice_to_pipe(struct pipe_inode_info *pipe,
>  {
>  	unsigned int spd_pages = spd->nr_pages;
>  	int ret, do_wakeup, page_nr;
> +	struct vm_area_struct *vma;
> +	unsigned long user_start, user_end;
>  
>  	ret = 0;
>  	do_wakeup = 0;
>  	page_nr = 0;
> +	vma = NULL;
> +	user_start = user_end = 0;
>  
>  	pipe_lock(pipe);
> +	down_read(&current->mm->mmap_sem);

Seems like you could take the mmap_sem only when GIFT and MOVE is set.
Maybe it won't help that much for performance but at least serve as
documenting the reason it's needed?

Vlastimil

>  	for (;;) {
>  		if (!pipe->readers) {
> @@ -212,8 +217,44 @@ ssize_t splice_to_pipe(struct pipe_inode_info *pipe,
>  			buf->len = spd->partial[page_nr].len;
>  			buf->private = spd->partial[page_nr].private;
>  			buf->ops = spd->ops;
> -			if (spd->flags & SPLICE_F_GIFT)
> +			if (spd->flags & SPLICE_F_GIFT) {
> +				unsigned long useraddr =
> +						spd->partial[page_nr].useraddr;
> +
> +				if ((spd->flags & SPLICE_F_MOVE) &&
> +						!buf->offset &&
> +						(buf->len == PAGE_SIZE)) {
> +					/* Can move page aligned buf, gather
> +					 * requests to make a single
> +					 * zap_page_range() call per VMA
> +					 */
> +					if (vma && (useraddr == user_end) &&
> +						   ((useraddr + PAGE_SIZE) <=
> +						    vma->vm_end)) {
> +						/* same vma, no holes */
> +						user_end += PAGE_SIZE;
> +					} else {
> +						if (vma)
> +							zap_page_range(vma,
> +								user_start,
> +								(user_end -
> +								 user_start),
> +								NULL);
> +						vma = find_vma_intersection(
> +								current->mm,
> +								useraddr,
> +								(useraddr +
> +								 PAGE_SIZE));
> +						if (!IS_ERR_OR_NULL(vma)) {
> +							user_start = useraddr;
> +							user_end = (useraddr +
> +								    PAGE_SIZE);
> +						} else
> +							vma = NULL;
> +					}
> +				}
>  				buf->flags |= PIPE_BUF_FLAG_GIFT;
> +			}
>  
>  			pipe->nrbufs++;
>  			page_nr++;
> @@ -255,6 +296,10 @@ ssize_t splice_to_pipe(struct pipe_inode_info *pipe,
>  		pipe->waiting_writers--;
>  	}
>  
> +	if (vma)
> +		zap_page_range(vma, user_start, (user_end - user_start), NULL);
> +
> +	up_read(&current->mm->mmap_sem);
>  	pipe_unlock(pipe);
>  
>  	if (do_wakeup)
> @@ -485,6 +530,7 @@ fill_it:
>  
>  		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;
>  };
>  
>  /*
> 

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