Re: [PATCH] lazy freeing of memory through MADV_FREE

[Rik van Riel <riel@redhat.com>]

Nick Piggin wrote:

> > It looks like the tlb flushes (and IPIs) from zap_pte_range()
> > could have been the problem.  They're gone now.
>
> I guess it is a good idea to batch these things. But can you
> do that on all architectures? What happens if your tlb flush
> happens after another thread already accesses it again, or
> after it subsequently gets removed from the address space via
> another CPU?

I have thought about this a lot tonight, and have come to the conclusion
that they are ok.

The reason is simple:

1) we do the TLB flush before we return from the
   madvise(MADV_FREE) syscall.

2) anything that accessess the pages between the start
   and end of the MADV_FREE procedure does not know in
   which order we go through the pages, so it could hit
   a page either before or after we get to processing
   it

3) because of this, we can treat any such accesses as
   happening simultaneously with the MADV_FREE and
   as illegal, aka undefined behaviour territory and
   we do not need to worry about them

4) because we flush the tlb before releasing the page
   table lock, other CPUs cannot remove this page from
   the address space - they will block on the page
   table lock before looking at this pte

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