Review request (hs24): 8007074: SIGSEGV at ParMarkBitMap::verify_clear()

[Florian Weimer]

On 07/02/2013 06:57 PM, Stefan Karlsson wrote:
> When the JVM starts it reserves a memory area for the entire Java heap.
> We use mmap(...MAP_NORESERVE...) to reserve a contiguous chunk of memory
> that no other
> subsystem of the JVM, or Java program, will be allowed to mmap into.
>
> The reservation of the memory only reflects the maximum possible heap
> size, but often a smaller heap size is used if the memory pressure is
> low. The part of
> the heap that is actually used is committed with mmap(...MAP_FIXED...).

Since you mention this, here's a really old pet peeve of mine.

This does not work as intended.  mmap(...MAP_NORESERVE...) actually 
commits the memory.  The correct way to reserve address space on Linux 
is to drop MAP_NORESERVE and map the memory with PROT_NONE. 
Subsequently, you can commit the memory using mprotect() and suitable 
protection flags.  This way, the default heap size will no longer cause 
failures with vm.overcommit_memory=2.

Would you be interested in a patch that improves matters in this area?

(Note that this only applies to Linux, not necessarily other systems.)

-- 
Florian Weimer / Red Hat Product Security Team