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

[Robert C Jennings <rcj@xxxxxxxxxxxxxxxxxx>]

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@xxxxxxxxx>
Signed-off-by: Robert C Jennings <rcj@xxxxxxxxxxxxxxxxxx>
---
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);
 
 	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;
 };
 
 /*
-- 
1.8.1.2

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