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Re: [Qemu-devel] [PATCH v2 2/3] block/fleecing-filter: new filter driver

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-devel] [PATCH v2 2/3] block/fleecing-filter: new filter driver for fleecing
Date: Mon, 2 Jul 2018 14:27:01 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

02.07.2018 09:35, Fam Zheng wrote:
On Fri, 06/29 12:24, Eric Blake wrote:
On 06/29/2018 10:15 AM, Vladimir Sementsov-Ogievskiy wrote:
We need to synchronize backup job with reading from fleecing image
like it was done in block/replication.c.

Otherwise, the following situation is theoretically possible:

Grammar suggestions:

1. client start reading
client starts reading

2. client understand, that there is no corresponding cluster in
     fleecing image
3. client is going to read from backing file (i.e. active image)
client sees that no corresponding cluster has been allocated in the fleecing
image, so the request is forwarded to the backing file

4. guest writes to active image
5. this write is stopped by backup(sync=none) and cluster is copied to
     fleecing image
6. guest write continues...
7. and client reads _new_ (or partly new) date from active image
Interesting race. Can it actually happen, or does our read code already
serialize writes to the same area while a read is underway?
Yes, I wonder why wait_serialising_requests() is not enough. If it's possible,
can we have a test case (with help of blkdebug, for example)?

Hmm, only unaligned and COPY_ON_READ requests are marked serializing.. So If I understand correctly, nothing special prevents intersection, it's a problem of the guest. But in backup case, it's our problem.

In short, I see what problem you are claiming exists: the moment the client
starts reading from the backing file, that portion of the backing file must
remain unchanged until after the client is done reading.  But I don't know
enough details of the block layer to know if this is actually a problem, or
if adding the new filter is just overhead.

Best regards,

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