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Re: [Qemu-block] [PATCH v7 4/9] block: treat BDRV_REQ_ALLOCATE as serial

From: Anton Nefedov
Subject: Re: [Qemu-block] [PATCH v7 4/9] block: treat BDRV_REQ_ALLOCATE as serialising
Date: Tue, 30 Jan 2018 15:36:26 +0300
User-agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2

On 29/1/2018 10:48 PM, Max Reitz wrote:
On 2018-01-18 18:49, Anton Nefedov wrote:
The idea is that ALLOCATE requests may overlap with other requests.
Reuse the existing block layer infrastructure for serialising requests.
Use the following approach:
   - mark ALLOCATE serialising, so subsequent requests to the area wait
   - ALLOCATE request itself must never wait if another request is in flight
     already. Return EAGAIN, let the caller reconsider.

Signed-off-by: Anton Nefedov <address@hidden>
Reviewed-by: Eric Blake <address@hidden>
  block/io.c | 27 +++++++++++++++++++--------
  1 file changed, 19 insertions(+), 8 deletions(-)

The basic principle looks good to me.

diff --git a/block/io.c b/block/io.c
index cf2f84c..4b0d34f 100644
--- a/block/io.c
+++ b/block/io.c


@@ -1717,7 +1728,7 @@ int coroutine_fn bdrv_co_pwritev(BdrvChild *child,
          struct iovec head_iov;
mark_request_serialising(&req, align);
-        wait_serialising_requests(&req);
+        wait_serialising_requests(&req, false);

What if someone calls bdrv_co_pwritev() with BDRV_REQ_ZERO_WRITE |


    assert(!(qiov && (flags & BDRV_REQ_ALLOCATE)));

will fail or bdrv_co_do_zero_pwritev() will be used.

.. Then this should do exactly the same as
bdrv_co_do_zero_pwritev(), which it currently does not -- besides this
serialization, this includes returning -ENOTSUP if there is a head or
tail to write.

Another question is if that assertion is ok.
In other words: should (qiov!=NULL && REQ_ALLOCATE) be a valid case?
e.g. with qiov filled with zeroes?

I'd rather document that not supported (and leave the assertion).

Actually, even (qiov!=NULL && REQ_ZERO_WRITE) looks kind of
unsupported/broken? Alignment code in bdrv_co_pwritev() zeroes out the
head and tail by passing the flag down bdrv_aligned_pwritev()

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