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Re: [PATCH] Re: [dm-devel] kmalloc after down_write?



On Friday 19 November 2004 2:27 pm, Alasdair G Kergon wrote:
> On Fri, Nov 19, 2004 at 02:16:52PM -0600, Kevin Corry wrote:
> > In register_snapshot(), move the kmalloc() outside the _origins_lock.
>
> Maybe - but snapshots and memory allocations don't mix particularly well,
> and these are complex trade-offs, so for now I'd rather leave this code
> as it is (i.e. not introduce new allocations), until there's strong
> evidence one way or the other e.g. from testing or more detailed code
> analysis.

I wasn't implying you needed to (or even should) accept that patch. :) Just 
put it out there as an example of how to reduce the time spent holding that 
lock without having to drop and reaquire it. I guess I should have marked my 
response as [RFC|PATCH].

Personally, I think this is one area where we really don't have to worry about 
lock contention. It's very unlikely in real-world scenarios that two 
different processes are going to be creating snapshots at the same time. 
Thinking about EVMS, I know there's no way to activate two snapshots 
simultaneously. Thinking about LVM2, you'd have to run two copies of lvcreate 
at the same point in time, which also seems kind of hard to imagine in 
practice.

Thus, I'd say the patch is mostly unnecessary.

-- 
Kevin Corry
kevcorry us ibm com
http://evms.sourceforge.net/


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