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From: | Anton Nefedov |
Subject: | Re: [Qemu-devel] [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 |BDRV_REQ_ALLOCATE?
Either 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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