dma-buf: system_heap: use larger contiguous mappings instead of per-page mmap

dma-buf: system_heap: use larger contiguous mappings instead of per-page mmap

We can allocate high-order pages, but mapping them one by
one is inefficient. This patch changes the code to map
as large a chunk as possible. The code looks somewhat
complicated mainly because supporting mmap with a
non-zero offset is a bit tricky.

Using the micro-benchmark below, we see that mmap becomes
35X faster:

  #include <stdio.h>
  #include <fcntl.h>
  #include <linux/dma-heap.h>
  #include <sys/ioctl.h>
  #include <sys/mman.h>
  #include <time.h>
  #include <unistd.h>
  #include <stdlib.h>

  #define SIZE   (512UL * 1024 * 1024)
  #define PAGE   4096
  #define STRIDE (PAGE/sizeof(int))
  #define PAGES  (SIZE/PAGE)

  int main(void) {
      int heap = open("/dev/dma_heap/system", O_RDONLY);
      struct dma_heap_allocation_data d =
            { .len = SIZE, .fd_flags = O_RDWR|O_CLOEXEC };
      ioctl(heap, DMA_HEAP_IOCTL_ALLOC, &d);

      struct timespec t0, t1;
      clock_gettime(CLOCK_MONOTONIC, &t0);
      int *p = mmap(NULL, SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, d.fd, 0);
      clock_gettime(CLOCK_MONOTONIC, &t1);

      for (int i = 0; i < PAGES; i++) p[i*STRIDE] = i;
      for (int i = 0; i < PAGES; i++)
          if (p[i*STRIDE] != i) {
              fprintf(stderr, "mismatch at page %d\n", i);
              exit(1);
          }

      long ns = (t1.tv_sec-t0.tv_sec)*1000000000L +
                (t1.tv_nsec-t0.tv_nsec);
      printf("mmap 512MB took %.3f us, verify OK\n", ns/1000.0);
      return 0;
  }

W/ patch:

~ # ./a.out
mmap 512MB took 200266.000 us, verify OK
~ # ./a.out
mmap 512MB took 198151.000 us, verify OK
~ # ./a.out
mmap 512MB took 197069.000 us, verify OK
~ # ./a.out
mmap 512MB took 196781.000 us, verify OK
~ # ./a.out
mmap 512MB took 198102.000 us, verify OK
~ # ./a.out
mmap 512MB took 195552.000 us, verify OK

W/o patch:

~ # ./a.out
mmap 512MB took 6987470.000 us, verify OK
~ # ./a.out
mmap 512MB took 6970739.000 us, verify OK
~ # ./a.out
mmap 512MB took 6984383.000 us, verify OK
~ # ./a.out
mmap 512MB took 6971311.000 us, verify OK
~ # ./a.out
mmap 512MB took 6991680.000 us, verify OK

Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Acked-by: John Stultz <jstultz@google.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
  [sumits: correct from 3.5x to 35x]
Link: https://patch.msgid.link/20251021042022.47919-1-21cnbao@gmail.com