* David Hildenbrand (da...@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-da...@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 / dgilb...@redhat.com / Manchester, UK
