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[Qemu-devel] [PATCH] block: Don't lose FUA flag during ZERO_WRITE fallba

From: Eric Blake
Subject: [Qemu-devel] [PATCH] block: Don't lose FUA flag during ZERO_WRITE fallback
Date: Sat, 30 Apr 2016 15:48:56 -0600

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

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: address@hidden
Signed-off-by: Eric Blake <address@hidden>

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.

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