From mboxrd@z Thu Jan 1 00:00:00 1970 From: Robert Jennings Subject: [RFC PATCH 0/2] vmpslice support for zero-copy gifting of pages Date: Thu, 25 Jul 2013 12:21:44 -0500 Message-ID: <1374772906-21511-1-git-send-email-rcj@linux.vnet.ibm.com> Cc: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Alexander Viro , Rik van Riel , Andrea Arcangeli , Dave Hansen , Robert Jennings , Matt Helsley , Anthony Liguori , Michael Roth , Lei Li , Leonardo Garcia To: linux-kernel@vger.kernel.org Return-path: Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org