Segmentation fault when a custom chunk allocator returns a memory block larger than the chunk size

[meng]

Hi All,
I used the new chunk allocator feature to allocate memory from a fixed 2G
memory region. Nevertheless, I got a seg. fault.

The flow of my code is as following:  I first use "arenas.extend" mallctl
to create a custom arena. Then I defined custom chunk_alloc() and
chunk_dalloc() on this arena. In the initialization phase of my code, I use
mmap() to reserve a memory region of size 2^32. In the custom
chunk_alloc(), I return the pointer of the 2^32B memory region. Because
lg_chunk is 2^22, I thought this should be fine. But the program ran into
seg. fault within  arena_mapbits_unzeroed_set() called
by arena_chunk_init_hard().  On the other hand, if the mmap() reserved a
memory region of size 2^22, everything works fine.

My question is: why does the custom chunk_alloc() always expect a memory
block returned from mmap()/malloc() with the requested size equal to
lg_chunk? I can't figure out what wrong it could be if the returned block
is a multiple of lg_chunk

B.T.W. My code only uses mallocx() for a single 1024B buffer from the
custom. Memory alignment problem shouldn't exist.

Thanks for your help!

George Meng
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