[PATCH v2 0/2] vmpslice support for zero-copy gifting of pages

[Robert Jennings]

From: Robert C Jennings <rcj@xxxxxxxxxxxxxxxxxx>

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/2013-10/msg02787.html

Changes from V1:
 - Cleanup zap coalescing in splice_to_pipe for readability
 - Field added to struct partial_page in v1 was unnecessary, using
   private field instead.
 - Read-side code in pipe_to_user pulled out into a new function
 - Improved documentation of read-side flipping code
 - Fixed locking issue in read-size flipping code found by sparse
 - Updated vmsplice comments for vmsplice_to_user(),
   vmsplice_to_pipe, and vmsplice syscall
_______________________________________________________

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

 fs/splice.c | 159 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 150 insertions(+), 9 deletions(-)

-- 
1.8.1.2

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