On 10/25/2013 05:46 PM, Robert Jennings wrote: > 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. Hello, since this seems somewhat narrow use case for a syscall change, it would be helpful if you included a larger discussion of considered existing alternatives, with benchmark results justifying the changed syscall. E.g.: - Cross Memory Attach comes to mind as one alternative to vmsplice. Although it does perform a single copy, there are results suggesting zero-copy doesn't necessarily add that much gain: http://marc.info/?l=linux-mm&m=130105930902915&w=2 - Would it be possible for QEMU to use shared memory to begin with? Since you are already restricting this to page-aligned regions. Ideally the benchmark results would also include the THP support when complete. Thanks, Vlastimil > 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(-) > -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
