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Re: What prevents discarding a cluster during rewrite?

From: Kevin Wolf
Subject: Re: What prevents discarding a cluster during rewrite?
Date: Tue, 23 Feb 2021 11:35:34 +0100

Am 22.02.2021 um 22:42 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 23.02.2021 00:30, Vladimir Sementsov-Ogievskiy wrote:
> > Thinking of how to prevent dereferencing to zero (and discard) of
> > host cluster during flush of compressed cache (which I'm working on
> > now), I have a question.. What prevents it for normal writes?
> I have no idea about why didn't it fail for years.. May be, I'm
> missing something?

Ouch. I suppose the reason why we never ran into it is that afaik Linux
drains the queue before sending discard requests.

Of course, we still need to protect against this in QEMU. We can't just
unref a cluster that is still in use.

> I have idea of fixing: increase the refcount of host cluster before
> write to data_file (it's important to increase refacount in same
> s->lock critical section where we get host_offset) and dereference it
> after write.. It should help. Any thoughts?

It would cause metadata updates for rewrites. I don't know whether the
intermediate value would ever be written to disk, but at least we'd
rewrite the same refcounts as before. I don't think we want that.

Discard requests might be rare enough that not considering the cluster
at all could work. We could then take a reader CoRwlock during most
operations, and a writer lock for discard.

Actually, maybe s->lock should be this CoRwlock, and instead of dropping
it temporarily like we do now we would just upgrade and downgrade it as
needed. Maybe this would allow finer grained locking in other places,


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