[v2,0/9] RAM_NORESERVE, MAP_NORESERVE and hostmem "reserve" property

Message

David Hildenbrand March 5, 2021, 10:16 a.m. UTC

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 == 1") 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 private memfd, shared file-based and shared hugetlbfs
mappings.

virtio-mem features I am currently working on include:
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/memmory regions, such that we won't
   always expose a large, sparse memory region to the VM.
3. Handling virtio-mem requests via an iothread to not hold the BQL while
   populating/preallocating memory
4. Protecting unplugged memory e.g., using userfaultfd.
5. (resizeable allocations / optimized mmap handling when resizing RAM
    blocks)

Based-on: [e-mail]

v1 -> v2:
- Rebased to upstream and phs_mem_alloc simplifications
-- Upsteam added the "map_offset" parameter to many RAM allocation
   interfaces.
- "softmmu/physmem: Drop "shared" parameter from ram_block_add()"
-- Use local variable "shared"
- "memory: introduce RAM_NORESERVE and wire it up in qemu_ram_mmap()"
-- Simplify due to phs_mem_alloc changes
- "util/mmap-alloc: Support RAM_NORESERVE via MAP_NORESERVE"
-- Add a whole bunch of comments.
-- Exclude shared anonymous memory that QEMU doesn't use
-- Special-case readonly mappings

Cc: Peter Xu <[e-mail]>
Cc: "Michael S. Tsirkin" <[e-mail]>
Cc: Eduardo Habkost <[e-mail]>
Cc: "Dr. David Alan Gilbert" <[e-mail]>
Cc: Richard Henderson <[e-mail]>
Cc: Paolo Bonzini <[e-mail]>
Cc: Igor Mammedov <[e-mail]>
Cc: "Philippe Mathieu-Daudé" <[e-mail]>
Cc: Stefan Hajnoczi <[e-mail]>
Cc: Murilo Opsfelder Araujo <[e-mail]>
Cc: Greg Kurz <[e-mail]>
Cc: Liam Merwick <[e-mail]>


David Hildenbrand (9):
  softmmu/physmem: Drop "shared" parameter from ram_block_add()
  util/mmap-alloc: Factor out calculation of the pagesize for the guard
    page
  util/mmap-alloc: Factor out reserving of a memory region to
    mmap_reserve()
  util/mmap-alloc: Factor out activating of memory to mmap_activate()
  softmmu/memory: Pass ram_flags into qemu_ram_alloc_from_fd()
  softmmu/memory: Pass ram_flags into
    memory_region_init_ram_shared_nomigrate()
  memory: introduce RAM_NORESERVE and wire it up in qemu_ram_mmap()
  util/mmap-alloc: Support RAM_NORESERVE via MAP_NORESERVE
  hostmem: Wire up RAM_NORESERVE via "reserve" property

 backends/hostmem-file.c                       |  11 +-
 backends/hostmem-memfd.c                      |   8 +-
 backends/hostmem-ram.c                        |   7 +-
 backends/hostmem.c                            |  33 +++
 hw/m68k/next-cube.c                           |   4 +-
 hw/misc/ivshmem.c                             |   5 +-
 include/exec/cpu-common.h                     |   1 +
 include/exec/memory.h                         |  43 ++--
 include/exec/ram_addr.h                       |   9 +-
 include/qemu/mmap-alloc.h                     |   2 +
 include/qemu/osdep.h                          |   3 +-
 include/sysemu/hostmem.h                      |   2 +-
 migration/ram.c                               |   3 +-
 .../memory-region-housekeeping.cocci          |   8 +-
 softmmu/memory.c                              |  27 ++-
 softmmu/physmem.c                             |  41 ++--
 util/mmap-alloc.c                             | 220 ++++++++++++------
 util/oslib-posix.c                            |   6 +-
 util/oslib-win32.c                            |  13 +-
 19 files changed, 299 insertions(+), 147 deletions(-)