[RFC PATCH 0/2] vmpslice support for zero-copy gifting of pages

[RFC PATCH 0/2] vmpslice support for zero-copy gifting of pages

[Robert Jennings]

This patch set would add the ability to move anonymous user pages from one
process to another through vmsplice without copying data.  Moving pages
rather than copying is implemented for a narrow case in this RFC to meet
the needs of QEMU's usage (below).

Among the restrictions the source address and destination addresses must
be page aligned, the size argument must be a multiple of page size,
and by the time the reader calls vmsplice, the page must no longer be
mapped in the source.  If a move is not possible the code transparently
falls back to copying data.

This comes from work in QEMU[1] to migrate a VM from one QEMU instance
to another with minimal down-time for the VM.  This would allow for an
update of the QEMU executable under the VM.

New flag usage
This introduces use of the SPLICE_F_MOVE flag for vmsplice, previously
unused.  Proposed usage is as follows:

 Writer gifts pages to pipe, can not access original contents after gift:
    vmsplice(fd, iov, nr_segs, (SPLICE_F_GIFT | SPLICE_F_MOVE);
 Reader asks kernel to move pages from pipe to memory described by iovec:
    vmsplice(fd, iov, nr_segs, SPLICE_F_MOVE);

Moving pages rather than copying is implemented for a narrow case in
this RFC to meet the needs of QEMU's usage.  If a move is not possible
the code transparently falls back to copying data.

For older kernels the SPLICE_F_MOVE would be ignored and a copy would occur.

[1] QEMU localhost live migration: 
      http://lists.gnu.org/archive/html/qemu-devel/2013-06/msg02540.html
      http://lists.gnu.org/archive/html/qemu-devel/2013-06/msg02577.html
_______________________________________________________

 RFC: vmsplice unmap gifted pages for recipient
 RFC: Add limited zero copy to vmsplice

 fs/splice.c            | 88 +++++++++++++++++++++++++++++++++++++++++++++++++-
 include/linux/splice.h |  1 +
 2 files changed, 88 insertions(+), 1 deletion(-)

-- 
1.8.1.2

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

[Robert Jennings]

From: Matt Helsley <matthltc@us.ibm.com>

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 <matthltc@us.ibm.com>
Signed-off-by: Matt Helsley <matt.helsley@gmail.com>
Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
---
 fs/splice.c            | 25 ++++++++++++++++++++++++-
 include/linux/splice.h |  1 +
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/fs/splice.c b/fs/splice.c
index 3b7ee65..6aa964f 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -172,6 +172,18 @@ static void wakeup_pipe_readers(struct pipe_inode_info *pipe)
 	kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
 }
 
+static void zap_buf_page(unsigned long useraddr)
+{
+	struct vm_area_struct *vma;
+
+	down_read(&current->mm->mmap_sem);
+	vma = find_vma_intersection(current->mm, useraddr,
+			useraddr + PAGE_SIZE);
+	if (!IS_ERR_OR_NULL(vma))
+		zap_page_range(vma, useraddr, PAGE_SIZE, NULL);
+	up_read(&current->mm->mmap_sem);
+}
+
 /**
  * splice_to_pipe - fill passed data into a pipe
  * @pipe:	pipe to fill
@@ -212,8 +224,16 @@ 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 */
+					zap_buf_page(useraddr);
 				buf->flags |= PIPE_BUF_FLAG_GIFT;
+			}
 
 			pipe->nrbufs++;
 			page_nr++;
@@ -485,6 +505,7 @@ 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 +677,7 @@ 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 +1497,7 @@ 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;
 
 			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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Re: [RFC PATCH 1/2] vmsplice unmap gifted pages for recipient

[Dave Hansen]

On 07/25/2013 10:21 AM, Robert Jennings wrote:
> +static void zap_buf_page(unsigned long useraddr)
> +{
> +	struct vm_area_struct *vma;
> +
> +	down_read(&current->mm->mmap_sem);
> +	vma = find_vma_intersection(current->mm, useraddr,
> +			useraddr + PAGE_SIZE);
> +	if (!IS_ERR_OR_NULL(vma))
> +		zap_page_range(vma, useraddr, PAGE_SIZE, NULL);
> +	up_read(&current->mm->mmap_sem);
> +}
> +
>  /**
>   * splice_to_pipe - fill passed data into a pipe
>   * @pipe:	pipe to fill
> @@ -212,8 +224,16 @@ 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 */
> +					zap_buf_page(useraddr);
>  				buf->flags |= PIPE_BUF_FLAG_GIFT;
> +			}

There isn't quite enough context here, but is it going to do this
zap_buf_page() very often?  Seems a bit wasteful to do the up/down and
find_vma() every trip through the loop.

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

[Robert Jennings]

* Dave Hansen (dave@sr71.net) wrote:
> On 07/25/2013 10:21 AM, Robert Jennings wrote:
> > +static void zap_buf_page(unsigned long useraddr)
> > +{
> > +	struct vm_area_struct *vma;
> > +
> > +	down_read(&current->mm->mmap_sem);
> > +	vma = find_vma_intersection(current->mm, useraddr,
> > +			useraddr + PAGE_SIZE);
> > +	if (!IS_ERR_OR_NULL(vma))
> > +		zap_page_range(vma, useraddr, PAGE_SIZE, NULL);
> > +	up_read(&current->mm->mmap_sem);
> > +}
> > +
> >  /**
> >   * splice_to_pipe - fill passed data into a pipe
> >   * @pipe:	pipe to fill
> > @@ -212,8 +224,16 @@ 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 */
> > +					zap_buf_page(useraddr);
> >  				buf->flags |= PIPE_BUF_FLAG_GIFT;
> > +			}
> 
> There isn't quite enough context here, but is it going to do this
> zap_buf_page() very often?  Seems a bit wasteful to do the up/down and
> find_vma() every trip through the loop.

The call to zap_buf_page() is in a loop where each pipe buffer is being
processed, but in that loop we have a pipe_wait() where we schedule().
So as things are structured I don't have the ability to hold mmap_sem
for multiple find_vma() calls.

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

[Dave Hansen]

On 07/26/2013 08:16 AM, Robert Jennings wrote:
>>> > > +				if ((spd->flags & SPLICE_F_MOVE) &&
>>> > > +				    !buf->offset && (buf->len == PAGE_SIZE))
>>> > > +					/* Can move page aligned buf */
>>> > > +					zap_buf_page(useraddr);
>>> > >  				buf->flags |= PIPE_BUF_FLAG_GIFT;
>>> > > +			}
>> > 
>> > There isn't quite enough context here, but is it going to do this
>> > zap_buf_page() very often?  Seems a bit wasteful to do the up/down and
>> > find_vma() every trip through the loop.
> The call to zap_buf_page() is in a loop where each pipe buffer is being
> processed, but in that loop we have a pipe_wait() where we schedule().
> So as things are structured I don't have the ability to hold mmap_sem
> for multiple find_vma() calls.

You can hold a semaphore over a schedule(). :)

You could also theoretically hold mmap_sem and only drop it on actual
cases when you reschedule if you were afraid of holding mmap_sem for
long periods of time (even though it's a read).


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[RFC PATCH 2/2] Add limited zero copy to vmsplice

[Robert Jennings]

From: Matt Helsley <matthltc@us.ibm.com>

It is sometimes useful to move anonymous pages over a pipe rather than
save/swap them. Check the SPLICE_F_GIFT and SPLICE_F_MOVE flags to see
if userspace would like to move such pages. This differs from plain
SPLICE_F_GIFT in that the memory written to the pipe will no longer
have the same contents as the original -- it effectively faults in new,
empty anonymous pages.

On the read side the page written to the pipe will be copied unless
SPLICE_F_MOVE is used. Otherwise copying will be performed and the page
will be reclaimed. Note that so long as there is a mapping to the page
copies will be done instead because rmap will have upped the map count for
each anonymous mapping; this can happen do to fork(), for example. This
is necessary because moving the page will usually change the anonymous
page's nonlinear index and that can only be done if it's unmapped.

Signed-off-by: Matt Helsley <matthltc@us.ibm.com>
Signed-off-by: Matt Helsley <matt.helsley@gmail.com>
Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
---
 fs/splice.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/fs/splice.c b/fs/splice.c
index 6aa964f..0a715c3 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -32,6 +32,10 @@
 #include <linux/gfp.h>
 #include <linux/socket.h>
 #include <linux/compat.h>
+#include <linux/page-flags.h>
+#include <linux/hugetlb.h>
+#include <linux/ksm.h>
+#include <linux/swapops.h>
 #include "internal.h"
 
 /*
@@ -1536,6 +1540,65 @@ static int pipe_to_user(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
 	char *src;
 	int ret;
 
+	if (!buf->offset && (buf->len == PAGE_SIZE) &&
+	    (buf->flags & PIPE_BUF_FLAG_GIFT) && (sd->flags & SPLICE_F_MOVE)) {
+		struct page *page = buf->page;
+		struct mm_struct *mm;
+		struct vm_area_struct *vma;
+		spinlock_t *ptl;
+		pte_t *ptep, pte;
+		unsigned long useraddr;
+
+		if (!PageAnon(page))
+			goto copy;
+		if (PageCompound(page))
+			goto copy;
+		if (PageHuge(page) || PageTransHuge(page))
+			goto copy;
+		if (page_mapped(page))
+			goto copy;
+		useraddr = (unsigned long)sd->u.userptr;
+		mm = current->mm;
+
+		ret = -EAGAIN;
+		down_read(&mm->mmap_sem);
+		vma = find_vma_intersection(mm, useraddr, useraddr + PAGE_SIZE);
+		if (IS_ERR_OR_NULL(vma))
+			goto up_copy;
+		if (!vma->anon_vma) {
+			ret = anon_vma_prepare(vma);
+			if (ret)
+				goto up_copy;
+		}
+		zap_page_range(vma, useraddr, PAGE_SIZE, NULL);
+		ret = lock_page_killable(page);
+		if (ret)
+			goto up_copy;
+		ptep = get_locked_pte(mm, useraddr, &ptl);
+		if (!ptep)
+			goto unlock_up_copy;
+		pte = *ptep;
+		if (pte_present(pte))
+			goto unlock_up_copy;
+		get_page(page);
+		page_add_anon_rmap(page, vma, useraddr);
+		pte = mk_pte(page, vma->vm_page_prot);
+		set_pte_at(mm, useraddr, ptep, pte);
+		update_mmu_cache(vma, useraddr, ptep);
+		pte_unmap_unlock(ptep, ptl);
+		ret = 0;
+unlock_up_copy:
+		unlock_page(page);
+up_copy:
+		up_read(&mm->mmap_sem);
+		if (!ret) {
+			ret = sd->len;
+			goto out;
+		}
+		/* else ret < 0 and we should fallback to copying */
+		VM_BUG_ON(ret > 0);
+	}
+copy:
 	/*
 	 * See if we can use the atomic maps, by prefaulting in the
 	 * pages and doing an atomic copy
-- 
1.8.1.2

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