Re: Review of Thomas's >2GB ext2fs proposal

[Thomas Bushnell BSG]

"Neal H. Walfield" <neal@cs.uml.edu> writes:

> You need to drain the cache once in a while.  You need some kind of
> strategy.

As I said, LRU is easy.

> Fine, but this is extra memory per region and it needs to be updated
> fairly often.  This is all gotten rid of when we just clear a mapping
> when the kernel does.

You still need the ref count!  Where are you going to put it?

> Of course you are right and I have taken this into account.  If a
> mapping has user references, the mapping is not removed.  It is likely
> that the page will be pulled back into memory in the near future
> anyways because of the user mapping.  But likely is certainly not
> enough of a reassurance.  So, in the case where the mapping cache is
> completely full and we need to force some pages out, we search for
> pages that have no user references.  Clearly this is an unoptimal
> eviction strategy, however, it should be rare enough satisfies the
> correctness requirement.

I thought you always allowed the kernel to do the pageouts....this is
additionaly complexity if you are sometimes forcing pages out.

> > > The interesting corner case is what happens if there are user
> > > references and the kernel drops the page.  Well, in this case, we
> > > just leave the mapping in place based on the assumption that the
> > > page will be faulted back in momentarily (otherwise, why would there
> > > by a reference?).
> > 
> > This is incorrect.  You might be just about to drop the reference,
> > having already completed all the uses of the page, when the page gets
> > paged out.
> 
> Fine, it is not rigorously correct, however, leaving it in the cache,
> as I describe above, does not hurt the system.

Assuming that it does eventually get dropped, otherwise, you have a
memory leak.  The kernel isn't going to be handing it back, so how
will you check?

(I think the solution is to have a proper interlock when you drop the
last reference, but this is very tricky to get right.)