Re: [PATCH v1 0/9] RAM_NORESERVE, MAP_NORESERVE and hostmem "reserve" property

[David Hildenbrand <david@redhat.com>]

On 09.02.21 14:49, David Hildenbrand wrote:
> Some cleanups previously sent in other context (resizeable allocations),
> followed by RAM_NORESERVE, implementing it under POSIX using MAP_NORESERVE,
> and letting users configure it for memory backens using the "reserve"
> property (default: true).
> 
> MAP_NORESERVE under Linux has in the context of QEMU an effect on
> 1) Private anonymous memory
> -> memory-backend-ram,id=mem0,size=10G
> 2) Private file-based mappings
> -> memory-backend-file,id=mem0,size=10G,mem-path=/dev/shm/0
> 3) Private/shared hugetlbfs memory
> -> memory-backend-memfd,id=mem0,size=10G,hugetlb=on,hugetlbsize=2M
> 
> With MAP_NORESERVE/"reserve=off", we won't be reserving swap space (1/2) or
> huge pages (3) for the whole memory region.
> 
> The target use case is virtio-mem, which dynamically exposes memory
> inside a large, sparse memory area to the VM. MAP_NORESERVE tells the OS
> "this mapping might be very sparse". This essentially allows
> avoiding to set "/proc/sys/vm/overcommit_memory == 0") when using
> virtio-mem and also supporting hugetlbfs in the future.
> 
> virtio-mem currently only supports anonymous memory, in the future we want
> to also support shared file-based and shared hugetlbfs mappings. We most
> probably won't be supporting private mappings as they can end up behaving
> very weird when it comes to memory consumption.
> 
> Future work for virtio-mem I am currently working on includes
> 1. Introducing a prealloc option for virtio-mem (e.g., using fallocate()
>     when plugging blocks) to fail nicely when running out of
>     backing storage like huge pages.
> 2. Supporting resizable RAM block/memoryr egions, such that we won't always
>     expose a large, sparse memory region to the VM.
> 3. Protecting unplugged memory e.g., using userfaultfd.
> 4. (resizeable allocations / optimized mmap handling when resizing RAM
>      blocks)

Ping

-- 
Thanks,

David / dhildenb