On Sat, Aug 30, 2025 at 4:58 PM Barry Song <[email protected]> wrote:
>
> From: Barry Song <[email protected]>
>
> 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
> 3.5X faster:
...
It's been awhile since I've done mm things, so take it with a pinch of
salt, but this seems reasonable to me.
Though, one thought below...
> diff --git a/drivers/dma-buf/heaps/system_heap.c
> b/drivers/dma-buf/heaps/system_heap.c
> index bbe7881f1360..4c782fe33fd4 100644
> --- a/drivers/dma-buf/heaps/system_heap.c
> +++ b/drivers/dma-buf/heaps/system_heap.c
> @@ -186,20 +186,35 @@ static int system_heap_mmap(struct dma_buf *dmabuf,
> struct vm_area_struct *vma)
> struct system_heap_buffer *buffer = dmabuf->priv;
> struct sg_table *table = &buffer->sg_table;
> unsigned long addr = vma->vm_start;
> - struct sg_page_iter piter;
> - int ret;
> + unsigned long pgoff = vma->vm_pgoff;
> + struct scatterlist *sg;
> + int i, ret;
> +
> + for_each_sgtable_sg(table, sg, i) {
> + unsigned long n = sg->length >> PAGE_SHIFT;
>
> - for_each_sgtable_page(table, &piter, vma->vm_pgoff) {
> - struct page *page = sg_page_iter_page(&piter);
> + if (pgoff < n)
> + break;
> + pgoff -= n;
> + }
> +
> + for (; sg && addr < vma->vm_end; sg = sg_next(sg)) {
> + unsigned long n = (sg->length >> PAGE_SHIFT) - pgoff;
> + struct page *page = sg_page(sg) + pgoff;
> + unsigned long size = n << PAGE_SHIFT;
> +
> + if (addr + size > vma->vm_end)
> + size = vma->vm_end - addr;
>
> - ret = remap_pfn_range(vma, addr, page_to_pfn(page), PAGE_SIZE,
> - vma->vm_page_prot);
> + ret = remap_pfn_range(vma, addr, page_to_pfn(page),
> + size, vma->vm_page_prot);
It feels like this sort of mapping loop for higher order pages
wouldn't be a unique pattern to just this code. Would this be better
worked into a helper so it would be more generally usable?
Otherwise,
Acked-by: John Stultz <[email protected]>
thanks
-john
