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

[Vlastimil Babka <vbabka@suse.cz>]

On 10/17/2013 03:48 PM, Robert Jennings wrote:
> * Vlastimil Babka (vbabka@suse.cz) wrote:
>> 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
>>
> 
> I had been doing that previously but moving this outside the loop and
> acquiring it once did improve performance.  I'll add a comment on
> down_read() as to the reason for taking this though.
> 
> -Rob

Hm perhaps in light of recent patches to reduce mmap_sem usage only to
really critical regions, maybe it really shouldn't be taken at all if
not 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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