linux-kernel - Re: [PATCH] dma-remap: fix dma_common_find

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Message-ID: <aT-jZyXGTVz-ziff@infradead.org>
Date: Sun, 14 Dec 2025 21:57:59 -0800
From: Christoph Hellwig <hch@...radead.org>
To: Andrei-Edward Popa <andrei.popa105@...oo.com>
Cc: m.szyprowski@...sung.com, robin.murphy@....com, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-remap: fix dma_common_find_pages() page lookup for
 offsets

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.