From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56216) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1awclV-00075s-BN for qemu-devel@nongnu.org; Sat, 30 Apr 2016 17:49:35 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1awclJ-00013m-Iw for qemu-devel@nongnu.org; Sat, 30 Apr 2016 17:49:23 -0400 Received: from resqmta-po-03v.sys.comcast.net ([2001:558:fe16:19:96:114:154:162]:60371) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1awclI-0000zO-7y for qemu-devel@nongnu.org; Sat, 30 Apr 2016 17:49:17 -0400 From: Eric Blake Date: Sat, 30 Apr 2016 15:48:56 -0600 Message-Id: <1462052936-16933-1-git-send-email-eblake@redhat.com> Subject: [Qemu-devel] [PATCH] block: Don't lose FUA flag during ZERO_WRITE fallback List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: qemu-stable@nongnu.org, Stefan Hajnoczi , Fam Zheng , Kevin Wolf , Max Reitz , "open list:Block I/O path" NBD has situations where it can support FUA but not ZERO_WRITE; when that happens, the generic block layer fallback was losing the FUA flag. The problem of losing flags unrelated to ZERO_WRITE has been latent in bdrv_co_do_write_zeroes() since aa7bfbff, but back then, it did not matter because there was no FUA flag. But ever since 93f5e6d8 added bdrv_co_writev_flags(), the loss of flags can impact correctness. Compare to commit 9eeb6dd, which got it right in bdrv_co_do_zero_pwritev(). Symptoms: I tested with qemu-io with default writethrough cache (which is supposed to use FUA semantics on every write), and targetted an NBD client connected to a server that intentionally did not advertise NBD_FLAG_SEND_FUA. When doing 'write 0 512', the NBD client sent two operations (NBD_CMD_WRITE then NBD_CMD_FLUSH) to get the fallback FUA semantics; but when doing 'write -z 0 512', the NBD client sent only NBD_CMD_WRITE; the missing flush meant that an ill-timed disconnect could leave the zeroes unflushed. CC: qemu-stable@nongnu.org Signed-off-by: Eric Blake --- As written, this patch applies to 2.7 on top of Kevin's block-next branch. Since it's (probably) too late for 2.6, we'll need to backport it to there, but the backport will have to use bdrv_co_writev_flags since 2.6 lacks bdrv_driver_pwritev(). block/io.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/block/io.c b/block/io.c index 0db1146..bd46e47 100644 --- a/block/io.c +++ b/block/io.c @@ -1213,7 +1213,8 @@ static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs, qemu_iovec_init_external(&qiov, &iov, 1); ret = bdrv_driver_pwritev(bs, sector_num * BDRV_SECTOR_SIZE, - num * BDRV_SECTOR_SIZE, &qiov, 0); + num * BDRV_SECTOR_SIZE, &qiov, + flags & ~BDRV_REQ_ZERO_WRITE); /* Keep bounce buffer around if it is big enough for all * all future requests. -- 2.5.5