Memory gurus,
We have an application where a driver allocates DMA-able memory. This DMA-able memory is mmap()ed to user-space. To conserve DMA memory only single pages are obtained using __get_dma_pages(GFP_KERNEL, 1) (one page at a time). These pages, that may be scattered all about, are mmap()ed into contiguous user data-space. The DMA engine uses a scatter-list so we can write DMA pages anywhere. They don't have to be contiguous.
Here's a catch. It would be nice to release those DMA pages when we don't need them. However, there doesn't appear to be any way for driver code to know when munmap() has been called and those DMA pages are available to be released.
How do I know that munmap() has been called? It turns out that if I release those pages before unmapping the user-mode, the system will crash. Therefore this could be a DOS attack if my driver ever allowed the DMA pages to be released.
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