Re: [PATCH] NOMMU: Separate out VMAs

[Bernd Schmidt <bernds_cb1@t-online.de>]

David Howells wrote:
> Bernd Schmidt <bernds_cb1@t-online.de> wrote:
>
> > In do_mmap_private, I've commented out the logic to free excess pages, as it
> > fragments terribly
>
> I wonder if there's a good heuristic for this.  The problem is that whilst
> not releasing excess pages _may_ seem like a good idea, if your system is
> something like a single persistent app, then it really is not.
>
> For instance, if such an app allocates a byte over 16MB (perhaps implicitly in
> the binfmt driver), then you'd completely waste a large chunk of RAM.  In the
> 16MB+1 case, the wastage would be a byte less than 16MB.

I think it would be good to have a mechanism to group free pages by 
purpose - so that if we break up a high-order page in order to allocate 
memory for process A, then the remaining pages remain in a special pool 
that the allocator will prefer to hand out only to process A.


> > Also, I think you're freeing high-order pages unaligned to
> > their order?
>
> Yeah, but some of the pages might still be in use when we want to release
> them.

Not following you here.  Is it valid to free an order-2 page that's not 
aligned at order-2?

> > In do_munmap, we can deal with freeing more than one vma.  I've not touched
> > the rb-tree logic in the shared file case, as I have no idea what it's trying
> > to do given that only exact matches are allowed.
>
> I'd generally rather not do this.  You can't use MAP_FIXED to request adjacent
> regions, so why should you anticipate there would be any?

Adjacent regions can happen by accident, and the uClibc malloc will try 
to merge free areas when they are adjacent - there's a lot of 
special-case code in there to prevent this on uClinux systems by 
essentially duplicating VMA tracking.  That's something we want to 
avoid, because it eats performance (especially in threaded apps due to 
additional locking).


Bernd
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