Am 24.06.21 um 07:34 schrieb Christoph Hellwig:

On Wed, Jun 23, 2021 at 10:00:29PM +0300, Oded Gabbay wrote:

...

I understand the argument and I agree that for the generic case, the top of the stack can't assume anything. Having said that, in this case the SGL is encapsulated inside a dma-buf object.

But the scatterlist is defined to have a valid page. If in dma-bufs you can't do that dmabufs are completely broken. Apparently the gpu folks can somehow live with that and deal with the pitfals, but for dma-buf users outside of their little fiefdom were they arbitrarily break rules it simply is not acceptable.

The key point is that accessing the underlying pages even when DMA-bufs are backed by system memory is illegal. Daniel even created a patch which mangles the page pointers in sg_tables used by DMA-buf to make sure that people don't try to use them.

So the conclusion is that using sg_table in the DMA-buf framework was just the wrong data structure and we should have invented a new one.

But then people would have complained that we have a duplicated infrastructure (which is essentially true).

My best plan to get out of this mess is that we change the DMA-buf interface to use an array of dma_addresses instead of the sg_table object and I have already been working on this actively the last few month.

Regards, Christian.