Re: [RFC] mmap(MAP_CONTIG)

On 10/03/2017 04:56 PM, Mike Kravetz wrote:

mmap(MAP_CONTIG) would have the following semantics:
- The entire mapping (length size) would be backed by physically contiguous
  pages.
- If 'length' physically contiguous pages can not be allocated, then mmap
  will fail.
- MAP_CONTIG only works with MAP_ANONYMOUS mappings.
- MAP_CONTIG will lock the associated pages in memory.  As such, the same
  privileges and limits that apply to mlock will also apply to MAP_CONTIG.
- A MAP_CONTIG mapping can not be expanded.

Do you also need to lock out the NUMA migration APIs somehow?  What
about KSM (or does it already ignore VM_LOCKED)?

- At fork time, private MAP_CONTIG mappings will be converted to regular
  (non-MAP_CONTIG) mapping in the child.  As such a COW fault in the child
  will not require a contiguous allocation.

Maybe we should just define it as acting as if it had MADV_DONTFORK set
on it, and also that it doesn't allow MADV_DONTFORK to be called on it.

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