Re: [PATCH v2 fixed 00/16] Ram blocks with resizable anonymous allocations under POSIX

[David Hildenbrand]

On 12.02.20 14:42, David Hildenbrand 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.
> 
> Especially, memory notifiers already handle resizing by first removing
> the old region, and then re-adding the resized region. prealloc is
> currently not possible with resizable ram blocks. mlock() should continue
> to work as is. Resizing is currently rare and must only happen on the
> start of an incoming migration, or during resets. No code path (except
> HAX and SEV ram block notifiers) should access memory outside of the usable
> range - and if we ever find one, that one has to be fixed (I did not
> identify any).
> 
> v1 -> v2:
> - Add "util: vfio-helpers: Fix qemu_vfio_close()"
> - Add "util: vfio-helpers: Remove Error parameter from
>        qemu_vfio_undo_mapping()"
> - Add "util: vfio-helpers: Factor out removal from
>        qemu_vfio_undo_mapping()"
> - "util/mmap-alloc: ..."
>  -- Minor changes due to review feedback (e.g., assert alignment, return
>     bool when resizing)
> - "util: vfio-helpers: Implement ram_block_resized()"
>  -- Reserve max_size in the IOVA address space.
>  -- On resize, undo old mapping and do new mapping. We can later implement
>     a new ioctl to resize the mapping directly.
> - "numa: Teach ram block notifiers about resizable ram blocks"
>  -- Pass size/max_size to ram block notifiers, which makes things easier an
>     cleaner
> - "exec: Ram blocks with resizable anonymous allocations under POSIX"
>  -- Adapt to new ram block notifiers
>  -- Shrink after notifying. Always trigger ram block notifiers on resizes
>  -- Add a safety net that all ram block notifiers registered at runtime
>     support resizes.
> 
> [1] https://lore.kernel.org/kvm/address@hidden/
> 
> David Hildenbrand (16):
>   util: vfio-helpers: Factor out and fix processing of existing ram
>     blocks
>   util: vfio-helpers: Fix qemu_vfio_close()
>   util: vfio-helpers: Remove Error parameter from
>     qemu_vfio_undo_mapping()
>   util: vfio-helpers: Factor out removal from qemu_vfio_undo_mapping()
>   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: Teach ram block notifiers about resizable ram blocks
>   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                     | 104 +++++++++++++++++++----
>  hw/core/numa.c             |  53 +++++++++++-
>  hw/i386/xen/xen-mapcache.c |   7 +-
>  include/exec/cpu-common.h  |   3 +
>  include/exec/memory.h      |   8 ++
>  include/exec/ramlist.h     |  14 +++-
>  include/qemu/mmap-alloc.h  |  21 +++--
>  include/qemu/osdep.h       |   6 +-
>  stubs/ram-block.c          |  20 -----
>  target/i386/hax-mem.c      |   5 +-
>  target/i386/sev.c          |  18 ++--
>  util/mmap-alloc.c          | 165 +++++++++++++++++++++++--------------
>  util/oslib-posix.c         |  37 ++++++++-
>  util/oslib-win32.c         |  14 ++++
>  util/trace-events          |   9 +-
>  util/vfio-helpers.c        | 145 +++++++++++++++++++++-----------
>  16 files changed, 450 insertions(+), 179 deletions(-)
> 

1. I will do resizable -> resizeable
2. I think migration might indeed need some care regarding
   max_length. We should never migrate anything beyond used_length. And
   if we receive something, it should be discarded. Will look into that.

-- 
Thanks,

David / dhildenb