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Re: [PATCH v2 fixed 00/16] Ram blocks with resizable anonymous allocatio


From: David Hildenbrand
Subject: Re: [PATCH v2 fixed 00/16] Ram blocks with resizable anonymous allocations under POSIX
Date: Thu, 13 Feb 2020 14:36:55 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 12.02.20 19:03, David Hildenbrand wrote:
> 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.

So I double checked and migration will never read or write beyond
used_length. Proper offset_in_ramblock() checks are in place. Will add
some cleanups to v3, though :)


-- 
Thanks,

David / dhildenb




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