Re: [PATCH] dma-remap: fix dma_common_find_pages() page lookup for offsets

[Christoph Hellwig]

On Fri, Dec 12, 2025 at 10:09:14PM +0200, Andrei-Edward Popa wrote:
> dma_common_find_pages() previously assumed that the CPU virtual address
> always pointed to the start of a DMA-coherent allocation. This fails when
> memory is allocated via dma_alloc_attrs() without DMA_ATTR_FORCE_CONTIGUOUS
> and then subdivided into smaller blocks using a gen_pool, relevant only
> when an IOMMU is enabled.
> 
> In such cases, userspace may request a mapping via dma_mmap_attrs()
> for a CPU address that is offset inside the original allocation. The
> previous code could return the wrong struct page pointer.

No, you can't mmap part of a dma coherent allocation.  What caller is
trying to do this?  It needs to be fixed instead.