Re: [PATCH][RFC] fast file mapping for loop

[Jens Axboe]

On Wed, Jan 09 2008, Christoph Hellwig wrote:
> On Wed, Jan 09, 2008 at 09:52:32AM +0100, Jens Axboe wrote:
> > - The file block mappings must not change while loop is using the file.
> >   This means that we have to ensure exclusive access to the file and
> >   this is the bit that is currently missing in the implementation. It
> >   would be nice if we could just do this via open(), ideas welcome...
> 
> And the way this is done is simply broken.  It means you have to get
> rid of things like delayed or unwritten hands beforehand, it'll be
> a complete pain for COW or non-block backed filesystems.

COW is not that hard to handle, you just need to be notified of moving
blocks. If you view the patch as just a tighter integration between loop
and fs, I don't think it's necessarily that broken.

I did consider these cases, and it can be done with the existing
approach.

> The right way to do this is to allow direct I/O from kernel sources
> where the filesystem is in-charge of submitting the actual I/O after
> the pages are handed to it.  I think Peter Zijlstra has been looking
> into something like that for swap over nfs.

That does sound like a nice approach, but a lot more work. It'll behave
differently too, the advantage of what I proposed is that it behaves
like a real device.

I'm not asking you to love it (in fact I knew some people would complain
about this approach and I understand why), just tossing it out there to
get things rolling. If we end up doing it differently I don't really
care, I'm not married to any solution but merely wish to solve a
problem. If that ends up being solved differently, the outcome is the
same to me.

-- 
Jens Axboe

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