On Linux, direct memory is a memfd mapped MAP_SHARED. amdgpu accepts only
private anonymous memory for a host pointer import (libdrm sets
AMDGPU_GEM_USERPTR_ANONONLY), so every import of direct memory failed with
VK_ERROR_INVALID_EXTERNAL_HANDLE and its ranges were copied.
When a host pointer import fails, the range's memfd slice is turned into a
dma-buf through /dev/udmabuf and imported with VK_EXT_external_memory_dma_buf.
The direct memory backing is sealed against shrinking and growing, as
udmabuf requires. libkernel reports the backing of a guest range through
GuestArenaSharedBacking. Without /dev/udmabuf, or past its size limit, the
range is copied as before. APS5_TRACE_HOST_IMPORT now prints the mappings
behind a failed import on Linux too.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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