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On Linux every CPU-visible pool had a host-visible shadow copy, because amdgpu treats a host-pointer import as a userptr object that it looks up again at every submission and invalidates whenever the write watch or a freeze re-protects its pages. Now the watched reservations are shared mappings of one memfd, /dev/udmabuf makes dma-bufs of page ranges of it, and the device imports them (VK_KHR_external_memory_fd, VK_EXT_external_memory_dma_buf). A dma-buf pins its pages once and is not tied to the process's page tables, so protection changes leave the GPU's mapping alone, and nothing is copied.
RADV puts every imported memory object into every submission, so pools share their imports: guest addresses are cut into 64 MiB spans (NX2NAVI_DMABUF_MB), and each pool is a buffer bound into its span's memory at its offset. A new span is cut back at the spans already there, so spans overlap only where a pool itself lies across an older span's edge, since amdgpu counts every span's pages in its GTT use. A decommit punches the pages out of the memfd, or zeroes them in place where a dma-buf pins them, and the pages a freed span pinned that nothing committed go back to the system.
It is on by default where /dev/udmabuf opens, udmabuf's size_limit_mb is 1024 or more, the device has both extensions and NVIDIA's kernel module is not loaded. A start-up probe imports one page as the pools will be; any refusal keeps the shadow copies, and the log says why. NX2NAVI_DMABUF=0 keeps the copies. tests/platform_dmabuf_test.c checks the shared mapping without a GPU, and docs/building.md ("Memory") describes the imports and what they need.
RX 6800 XT (RADV, Mesa 26.2.6), one build, interleaved pairs with NX2NAVI_DMABUF=1 and 0: Kokiri Forest 60.7 against 40.3 fps (3 pairs, +51%, standard error 0.4), 1% low 50.9 against 34.0, frames over 25 ms 5 against 768; Hyrule Field 55.7 against 42.4 fps (2 pairs, +31%, standard error 0.4). No pool was left without a span, no span was refused, and --probe-vkmem passes with and without the imports.