what is the size limit for mmap

Question

I am using mmap() to map a shared memory object to a process. My question has two parts:

1) what is the size limit for mmap() to a linux process? (is there such limit?)

2) after the process running a while, I think the process virtual memory address space will be somehow fragmented. Will this impact the max size I can do mmap() in this process?

The linux kernel used is 2.6.27. The size of the shared memory object is around 32MB. I am trying to access what is the possibility that mmap() fails with such shared memory object due to no enough virtual address space.

Answer 1

There is no restriction on mmap size but would depend on the existing address space used by the given process. But it is highly suggested that you dont mmap to a large contiguous virtual address space. Another suggestion would be to use the mmap "just-in-time" when a specific physical address (or a device address) is accessed and unmap once done

Answer 2

/proc/sys/vm/overcommit_memory controls the maximum on Linux

I have explained this in detail at: maximum memory which malloc can allocate

Basically, the value of 1 allows arbitrary virtual allocations, while 0 and 2 have more complicated limit calculations.