Re: [PATCH 1/4] NTB: ntb_transport: Handle remapped contiguous region in vmalloc space

[Logan Gunthorpe]

On 2025-10-26 18:43, Koichiro Den wrote:
> The RX buffer virtual address may reside in vmalloc space depending on
> the allocation path, where virt_to_page() is invalid.
> 
> Use a helper that chooses vmalloc_to_page() or virt_to_page() as
> appropriate. This is safe since the buffer is guaranteed to be
> physically contiguous.

I think this statement needs some explanation.

vmalloc memory is generally not contiguous and using vmalloc_to_page()
like this seems very questionable.

I did a very quick look and found that "offset" may come from
dma_alloc_attrs() which can also return coherent memory that would be in
vmalloc space and would be contiguous.

However, in my cursory look, it appears that the kernel address returned
by dma_alloc_attrs() is eventually passed to dma_map_page() in order to
obtain the dma address a second time. This is really ugly, and almost
certainly not expected by the dma layer.

This requires a bit of a change, but it seems to me that if
dma_alloc_attrs() is used, the dma address it returns should be used
directly and a second map should be avoided completely. Then we wouldn't
need the unusual use of vmalloc_to_page().

At the very least, I think these issues need to be mentioned in the
commit message.

Logan