Re: [RFC] mmap(MAP_CONTIG)

[Dave Hansen]

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.