On 5/8/24 08:16, Daniel Vetter wrote:
On Tue, May 07, 2024 at 08:32:47PM -0300, Jason Gunthorpe wrote:
On Tue, May 07, 2024 at 08:35:37PM +0100, Pavel Begunkov wrote:
On 5/7/24 18:56, Jason Gunthorpe wrote:
On Tue, May 07, 2024 at 06:25:52PM +0100, Pavel Begunkov wrote:
On 5/7/24 17:48, Jason Gunthorpe wrote:
On Tue, May 07, 2024 at 09:42:05AM -0700, Mina Almasry wrote:
1. Align with devmem TCP to use udmabuf for your io_uring memory. I
think in the past you said it's a uapi you don't link but in the face
of this pushback you may want to reconsider.
dmabuf does not force a uapi, you can acquire your pages however you
want and wrap them up in a dmabuf. No uapi at all.
The point is that dmabuf already provides ops that do basically what
is needed here. We don't need ops calling ops just because dmabuf's
ops are not understsood or not perfect. Fixup dmabuf.
Those ops, for example, are used to efficiently return used buffers
back to the kernel, which is uapi, I don't see how dmabuf can be
fixed up to cover it.
Sure, but that doesn't mean you can't use dma buf for the other parts
of the flow. The per-page lifetime is a different topic than the
refcounting and access of the entire bulk of memory.
Ok, so if we're leaving uapi (and ops) and keep per page/sub-buffer as
is, the rest is resolving uptr -> pages, and passing it to page pool in
a convenient to page pool format (net_iov).
I'm not going to pretend to know about page pool details, but dmabuf
is the way to get the bulk of pages into a pool within the net stack's
allocator and keep that bulk properly refcounted while.
An object like dmabuf is needed for the general case because there are
not going to be per-page references or otherwise available.
What you seem to want is to alter how the actual allocation flow works
from that bulk of memory and delay the free. It seems like a different
topic to me, and honestly hacking into the allocator free function
seems a bit weird..
Also I don't see how it's an argument against dma-buf as the interface for
It's not, neither I said it is, but it is an argument against removing
the network's page pool ops.
all these, because e.g. ttm internally does have a page pool because
depending upon allocator, that's indeed beneficial. Other drm drivers have
more buffer-based concepts for opportunistically memory around, usually
by marking buffers that are just kept as cache as purgeable (which is a
concept that goes all the way to opengl/vulkan).
Because in this case it solves nothing and helps with nothing, quite
the opposite. Just as well we can ask why NVMe doesn't wrap user pages
into a dmabuf while doing IO.
But these are all internals of the dma-buf exporter, the dma-buf api users
don't ever need to care.
-Sima
--
Pavel Begunkov
