On 5/7/24 17:23, Christoph Hellwig wrote:
On Tue, May 07, 2024 at 01:18:57PM -0300, Jason Gunthorpe wrote:
On Tue, May 07, 2024 at 05:05:12PM +0100, Pavel Begunkov wrote:
even in tree if you give them enough rope, and they should not have that rope when the only sensible options are page/folio based kernel memory (incuding large/huge folios) and dmabuf.
I believe there is at least one deep confusion here, considering you previously mentioned Keith's pre-mapping patches. The "hooks" are not that about in what format you pass memory, it's arguably the least interesting part for page pool, more or less it'd circulate whatever is given. It's more of how to have a better control over buffer lifetime and implement a buffer pool passing data to users and empty buffers back.
Isn't that more or less exactly what dmabuf is? Why do you need another almost dma-buf thing for another project?
That's the exact point I've been making since the last round of the series. We don't need to reinvent dmabuf poorly in every subsystem, but instead fix the odd parts in it and make it suitable for everyone.
Someone would need to elaborate how dma-buf is like that addition to page pool infra. The granularity here is usually 4K and less (hw dictated), what user receives cannot be guaranteed to be contiguous in memory. Having thousands of dma-buf instances is not an option, so a completion would need to include a range where data sits. Then who controls lifetime of buffers? If it's dma-buf, then at least it needs to track what sub-buffers are handed to user and what are currently in the kernel. How it would be accounted? ioctl_return_subrange(dmabuf, [range]), sounds like a bad idea for performance. To cover user memory it'd also need to be read from userspace, ioctl here wouldn't be an option, but let's say it's somehow done in the kernel.
That's not all the list, but in short, even though I haven't been following dma-buf developments too closely, I have hard time seeing how it can be a replacement here.