Bernd Schmidt <[email protected]> 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.
> and causes a simple
> while true; do cat /proc/buddyinfo; done
> loop to go oom.
Are you sure it's not just another leak?
> 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.
> In shrink_vma, we must save the mm across calls to remove_vma_from_mm (oops
> when telnetting into the box).
I'll have a look, but I don't see that.
> 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?
> It still does not survive my mmap stress-tester, so I'll keep looking.
Thanks.
> Why do we need vm_regions for anonymous memory? Wouldn't it be enough to just
> have a VMA?
It makes it simpler to have a common way of allocating memory for both anon
regions and file regions.
David
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