Re: [PATCH v1 00/13] Ram blocks with resizable anonymous allocations under POSIX

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* David Hildenbrand (david@redhat.com) wrote:
> We already allow resizable ram blocks for anonymous memory, however, they
> are not actually resized. All memory is mmaped() R/W, including the memory
> exceeding the used_length, up to the max_length.
> 
> When resizing, effectively only the boundary is moved. Implement actually
> resizable anonymous allocations and make use of them in resizable ram
> blocks when possible. Memory exceeding the used_length will be
> inaccessible. Especially ram block notifiers require care.
> 
> Having actually resizable anonymous allocations (via mmap-hackery) allows
> to reserve a big region in virtual address space and grow the
> accessible/usable part on demand. Even if "/proc/sys/vm/overcommit_memory"
> is set to "never" under Linux, huge reservations will succeed. If there is
> not enough memory when resizing (to populate parts of the reserved region),
> trying to resize will fail. Only the actually used size is reserved in the
> OS.
> 
> E.g., virtio-mem [1] wants to reserve big resizable memory regions and
> grow the usable part on demand. I think this change is worth sending out
> individually. Accompanied by a bunch of minor fixes and cleanups.
> 
> [1] https://lore.kernel.org/kvm/20191212171137.13872-1-david@redhat.com/

There's a few bits I've not understood from skimming the patches:

  a) Am I correct in thinking you PROT_NONE the extra space so you can
gkrow/shrink it?
  b) What does kvm see - does it have a slot for the whole space or for
only the used space?
     I ask because we found with virtiofs/DAX experiments that on Power,
kvm gets upset if you give it a mapping with PROT_NONE.
     (That maybe less of an issue if you change the mapping after the
slot is created).

  c) It's interesting this is keyed off the RAMBlock notifiers - do
     memory_listener's on the address space the block is mapped into get
    triggered?  I'm wondering how vhost (and vhost-user) in particular
    see this.

Dave

> 
> David Hildenbrand (13):
>   util: vfio-helpers: Factor out and fix processing of existings ram
>     blocks
>   exec: Factor out setting ram settings (madvise ...) into
>     qemu_ram_apply_settings()
>   exec: Reuse qemu_ram_apply_settings() in qemu_ram_remap()
>   exec: Drop "shared" parameter from ram_block_add()
>   util/mmap-alloc: Factor out calculation of pagesize to mmap_pagesize()
>   util/mmap-alloc: Factor out reserving of a memory region to
>     mmap_reserve()
>   util/mmap-alloc: Factor out populating of memory to mmap_populate()
>   util/mmap-alloc: Prepare for resizable mmaps
>   util/mmap-alloc: Implement resizable mmaps
>   numa: Introduce ram_block_notify_resized() and
>     ram_block_notifiers_support_resize()
>   util: vfio-helpers: Implement ram_block_resized()
>   util: oslib: Resizable anonymous allocations under POSIX
>   exec: Ram blocks with resizable anonymous allocations under POSIX
> 
>  exec.c                    |  99 ++++++++++++++++++----
>  hw/core/numa.c            |  39 +++++++++
>  include/exec/cpu-common.h |   3 +
>  include/exec/memory.h     |   8 ++
>  include/exec/ramlist.h    |   4 +
>  include/qemu/mmap-alloc.h |  21 +++--
>  include/qemu/osdep.h      |   6 +-
>  stubs/ram-block.c         |  20 -----
>  util/mmap-alloc.c         | 168 ++++++++++++++++++++++++--------------
>  util/oslib-posix.c        |  37 ++++++++-
>  util/oslib-win32.c        |  14 ++++
>  util/trace-events         |   5 +-
>  util/vfio-helpers.c       |  33 ++++----
>  13 files changed, 331 insertions(+), 126 deletions(-)
> 
> -- 
> 2.24.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK