Main steps to load file data into dmabuf: 1. dmabuf_fd = dmabuf_alloc(len, heap_fd) 2. vaddr = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, dmabuf_fd, 0) 3. file_fd = open(file_path, O_RDONLY) 4. read(file_fd, vaddr, len)
dmabuf's attachment/map/fence model sets VM_PFNMAP for mmap, which lacks direct I/O support[1]. Buffer IO causes latency when loading large AI model files. My previous patch added dmabuf ioctl for Direct IO file operations, showing good performance on low-power CPUs[2][3]. Christian suggested using existing uAPIs (read/sendfile/splice/c_f_r) instead of new ioctls. sendfile/splice/c_f_r enable zero-copy via Direct IO for disk-disk/network: sendfile(skt_fd, disk_fd): [DISK]-DMA->[pipe(buf)]-DMA->[NIC] sendfile(dst_disk, src_disk): [DISK] -DMA-> [pipe(buf)] -DMA-> [DISK] Analysis shows existing uAPIs can't achieve zero-copy disk-to-dmabuf. Since dmabuf lacks file ops, using tmpfs for disk-to-tmpfs CPU analysis: | Method | CPU Copies | Key Overhead | |-------------------|------------|----------------------------| | 1. Buffer R+W | 2 | Alloc(cache) & 2 CPU copies| | 2. Direct R+W | 1 | GUP(usr_buf) & 1 CPU copy | | 3. Mmap+Buffer R | 1 | Alloc(cache) & 1 CPU copy | | 4. Mmap+Direct R | 0 | GUP(mem_buf) ~50% CPU | | 5. Buffer Sendfile| 1 | Alloc(cache) & 1 CPU copy | | 6. Direct Sendfile| 1 | Small pipe, high IRQ | | 7. Buffer Splice | 1 | Alloc(cache) & 1 CPU copy | | 8. Direct Splice | 1 | Larger pipe buffer | | 9. c_f_r | N/A | Cross-FS blocked | GUP: get_user_page Alloc(cache): allocate page cache Data flows: 1. [DISK] -DMA-> [Alloc(cache)] -COPY-> [usr_buf] -COPY-> [MEM] 2. [DISK] -DMA-> [GUP(usr_buf)] -COPY-> [MEM] 3. [DISK] -DMA-> [Alloc(cache)] -COPY-> [mem_buf] 4. [DISK] -DMA-> [GUP(mem_buf)] 5. [DISK] -DMA-> [pipe(Alloc(cache))] -COPY-> [tmpfs page] 6. [DISK] -DMA-> [pipe(buf)] -COPY-> [tmpfs page] 7. [DISK] -DMA-> [big_pipe(Alloc(cache))] -COPY-> [tmpfs page] 8. [DISK] -DMA-> [big_pipe(buf)] -COPY-> [tmpfs page] 9. [DISK] -DMA-> [tmpfs page] (blocked) Key findings: - Buffer I/O requires page cache allocation and at least one CPU copy - Read+Write incurs excessive CPU copies and will no longer be analyzed. Future approaches will use Read instead of mmap+Read. - Mmap+Direct has zero copies but 50% GUP overhead, and dmabuf doesn't support - sendfile/splice require intermediate pipes, needing 1 CPU copy - c_f_r limitations: Cross-FS blocks + missing memory FS support Modifications: 1. Enable cross-FS c_f_r for memory file types 2. Add dmabuf c_f_r callbacks for [DISK]-DMA->[dmabuf] 3. Test tmpfs c_f_r locally only (no upstream) due to lock_page deadlock risks Performance (1GHz CPU, UFS4@4GB): 1. tmpfs(memfd) direct c_f_r(1197 MB/s): +15% vs mmap&read(1014) 2. udmabuf+memfd(2318 MB/s): +50% vs mmap&read(1457 MB/s) 3. dmabuf direct c_f_r(3405 MB/s): 260% faster than buffer IO(918 MB/s) 40% faster than udmabuf(2318 MB/s) | 32x32MB Read 1024MB |Creat-ms|Close-ms| I/O-ms|I/O-MB/s| I/O% |-------------------------|--------|--------|--------|--------|----- | 1)Beg dmabuf buffer R/W| 52 | 5 | 1170 | 918 | 100% | 2) udmabuf buffer R/W| 591 | 326 | 1281 | 838 | 91% | 3) memfd buffer R/W| 1 | 323 | 2370 | 453 | 49% | 4) memfd direct R/W| 1 | 321 | 1058 | 1014 | 110% | 5) memfd buffer sendfile| 1 | 329 | 1577 | 681 | 74% | 6) memfd direct sendfile| 1 | 327 | 2672 | 401 | 43% | 7) memfd buffer splice| 2 | 321 | 1729 | 621 | 67% | 8) memfd direct splice| 2 | 324 | 1528 | 702 | 76% | 9) memfd buffer c_f_r| 1 | 325 | 1586 | 677 | 73% |10) memfd direct c_f_r| 1 | 323 | 897 | 1197 | 130% |11) u+mfd buffer R/W| 609 | 344 | 2207 | 486 | 52% |12) u+mfd direct R/W| 580 | 342 | 737 | 1457 | 158% |13) u+mfd buffer sendfile| 582 | 343 | 1270 | 845 | 92% |14) u+mfd direct sendfile| 573 | 344 | 2254 | 476 | 51% |15) u+mfd buffer splice| 584 | 341 | 1202 | 893 | 97% |16) u+mfd direct splice| 564 | 340 | 851 | 1263 | 137% |17) u+mfd buffer c_f_r| 585 | 344 | 1244 | 863 | 94% |18) u+mfd direct c_f_r| 578 | 341 | 581 | 1848 | 201% |19) udmabuf buffer c_f_r| 585 | 328 | 1163 | 923 | 100% |20) udmabuf direct c_f_r| 579 | 328 | 464 | 2318 | 252% |21) dmabuf buffer c_f_r| 48 | 5 | 1058 | 1015 | 110% |22) dmabuf direct c_f_r| 48 | 5 | 316 | 3405 | 370% |23)End dmabuf buffer R/W| 48 | 5 | 1173 | 915 | 99% u+mfd = udma+memfd = udmabuf + pre-allocated memfd combo. Cache cleared during tests to simulate real-world large file loading. dmabuf file Use Cases: - Loading large AI models using dmabuf - Real-time data capture and storage with dmabuf - Persisting task snapshots in Android v3 -> v4: Add memory_copy_file_fops to simplify code and add FMODE_ODIRECT check Explicitly add dependency headers for udmabuf Simplify rw_file implementation in udmabuf/system_heaps Set FMODE_ODIRECT for dmabuf supporting Direct I/O v2 -> v3: [4] copy_file_range supports copying from disk files to memory files. Implement the copy_file_range callback functions for dmabuf/udmabuf. v1 -> v2: [3] Dma-buf exporter verify exclusive access to the dmabuf's sgtable. v1: [2] Reference: [1] https://lore.kernel.org/all/0393cf47-3fa2-4e32-8b3d-d5d5bdece...@amd.com [2] https://lore.kernel.org/all/20250513092803.2096-1-tao.wang...@honor.com [3] https://lore.kernel.org/all/20250516092148.12778-1-tao.wang...@honor.com [4] https://lore.kernel.org/all/20250530103941.11092-1-tao.wang...@honor.com wangtao (4): fs: allow cross-FS copy_file_range for memory file with direct I/O dmabuf: Implement copy_file_range callback for dmabuf direct I/O prep udmabuf: Implement udmabuf direct I/O dmabuf:system_heap Implement system_heap dmabuf direct I/O drivers/dma-buf/dma-buf.c | 32 +++++++++++++ drivers/dma-buf/heaps/system_heap.c | 69 +++++++++++++++++++++++++++++ drivers/dma-buf/udmabuf.c | 54 ++++++++++++++++++++++ fs/read_write.c | 64 +++++++++++++++++++++----- include/linux/dma-buf.h | 16 +++++++ include/linux/fs.h | 2 + 6 files changed, 225 insertions(+), 12 deletions(-) -- 2.17.1
