On Thu, Jan 14, 2021 at 1:03 AM Daniel Vetter daniel.vetter@ffwll.ch wrote:
On Wed, Jan 13, 2021 at 10:08 PM Chris Wilson chris@chris-wilson.co.uk wrote:
Quoting Daniel Vetter (2021-01-13 20:50:11)
On Wed, Jan 13, 2021 at 4:43 PM Chris Wilson chris@chris-wilson.co.uk wrote:
Quoting Daniel Vetter (2021-01-13 14:06:04)
We have too many people abusing the struct page they can get at but really shouldn't in importers. Aside from that the backing page might simply not exist (for dynamic p2p mappings) looking at it and using it e.g. for mmap can also wreak the page handling of the exporter completely. Importers really must go through the proper interface like dma_buf_mmap for everything.
If the exporter doesn't want to expose the struct page, why are they setting it in the exported sg_table?
You need to store it somewhere, otherwise the dma-api doesn't work. Essentially this achieves clearing/resetting the struct page pointer, without additional allocations somewhere, or tons of driver changes (since presumably the driver does keep track of the struct page somewhere too).
Only for mapping, and that's before the export -- if there's even a struct page to begin with.
Also as long as we have random importers looking at struct page we can't just remove it, or crashes everywhere. So it has to be some debug option you can disable.
Totally agreed that nothing generic can rely on pages being transported via dma-buf, and memfd is there if you do want a suitable transport. The one I don't know about is dma-buf heap, do both parties there consent to transport pages via the dma-buf? i.e. do they have special cases for import/export between heaps?
heaps shouldn't be any different wrt the interface exposed to importers. Adding John just in case I missed something.
I'm not aware of how this would be an issue right off for dma-buf heaps. Obviously there may be some corner cases with things like secure heaps, but I've not gotten to work on any of those yet and there's none in-tree. I did test out the patch on HiKey960 (using the cma and system heap for display and gpu buffers - admittedly not particularly complex) and didn't see any issues with it enabled.
I've added Suren and Hridya for more input but don't have any objections right off.
thanks -john