munmap() failure with ENOMEM with private anonymous mapping

Question

I have recently discovered that Linux does not guarantee that memory allocated with mmap can be freed with munmap if this leads to situation when n

Answer 1

One way to work around this problem on Linux is to mmap more that 1 page at once (e.g. 1 MB at a time), and also map a separator page after it. So, you actually call mmap on 257 pages of memory, then remap the last page with PROT_NONE, so that it cannot be accessed. This should defeat the VMA merging optimization in the kernel. Since you are allocating many pages at once, you should not run into the max mapping limit. The downside is you have to manually manage how you want to slice the large mmap.

As to your questions:

1. System calls can fail on any system for a variety of reasons. Documentation is not always complete.

2. You are allowed to munmap a part of a mmapd region as long as the address passed in lies on a page boundary, and the length argument is rounded up to the next multiple of the page size.