[Bug malloc/15321] malloc/free can't give the memory back to kernel when main_arena is discontinous

["fweimer at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=15321

--- Comment #10 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to Carlos O'Donell from comment #9)
> (In reply to ma.jiang from comment #8)
> > 	void *tt = mmap(mem[i-1]+1024*1024, 1024*213, PROT_READ |
> > PROT_WRITE,MAP_SHARED | MAP_ANONYMOUS, -1, 0);
> 
> This corrupts the arena and the chunk metadata by writing zero (if the
> kernel honours the address hint).


If it does, it's a kernel bug.  If the break point overlaps with the mapping,
the mmap call must fail because MAP_FIXED hasn't been specified.

Even without the hint, something like that could happen under virtual address
space pressure.

I think this needs further analysis.

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