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

[Jason Evans]

On Jul 28, 2014, at 2:17 PM, meng <xqmeng at gmail.com> wrote:
> 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.

Is the address you're returning from the custom chunk_alloc() aligned at a multiple of the chunk size?

Jason