On Tue, Jul 16, 2024 at 10:48:40AM +0800, Huan Yang wrote:
I just research the udmabuf, Please correct me if I'm wrong.
在 2024/7/15 20:32, Christian König 写道:
Am 15.07.24 um 11:11 schrieb Daniel Vetter:
On Thu, Jul 11, 2024 at 11:00:02AM +0200, Christian König wrote:
Am 11.07.24 um 09:42 schrieb Huan Yang:
Some user may need load file into dma-buf, current
way is:
1. allocate a dma-buf, get dma-buf fd
2. mmap dma-buf fd into vaddr
3. read(file_fd, vaddr, fsz)
This is too heavy if fsz reached to GB.
You need to describe a bit more why that is to heavy. I can only
assume you
need to save memory bandwidth and avoid the extra copy with the CPU.
This patch implement a feature called DMA_HEAP_IOCTL_ALLOC_READ_FILE.
User need to offer a file_fd which you want to load into
dma-buf, then,
it promise if you got a dma-buf fd, it will contains the file content.
Interesting idea, that has at least more potential than trying
to enable
direct I/O on mmap()ed DMA-bufs.
The approach with the new IOCTL might not work because it is a very
specialized use case.
But IIRC there was a copy_file_range callback in the file_operations
structure you could use for that. I'm just not sure when and how
that's used
with the copy_file_range() system call.
I'm not sure any of those help, because internally they're all still
based
on struct page (or maybe in the future on folios). And that's the thing
dma-buf can't give you, at least without peaking behind the curtain.
I think an entirely different option would be malloc+udmabuf. That
essentially handles the impendence-mismatch between direct I/O and
dma-buf
on the dma-buf side. The downside is that it'll make the permanently
pinned memory accounting and tracking issues even more apparent, but I
guess eventually we do need to sort that one out.
Oh, very good idea!
Just one minor correction: it's not malloc+udmabuf, but rather
create_memfd()+udmabuf.
Hm right, it's create_memfd() + mmap(memfd) + udmabuf
And you need to complete your direct I/O before creating the udmabuf
since that reference will prevent direct I/O from working.
udmabuf will pin all pages, so, if returned fd, can't trigger direct I/O
(same as dmabuf). So, must complete read before pin it.
Why does pinning prevent direct I/O? I haven't tested, but I'd expect the
rdma folks would be really annoyed if that's the case ...
But current way is use `memfd_pin_folios` to boost alloc and pin, so maybe
need suit it.
I currently doubt that the udmabuf solution is suitable for our
gigabyte-level read operations.
1. The current mmap operation uses faulting, so frequent page faults will be
triggered during reads, resulting in a lot of context switching overhead.
2. current udmabuf size limit is 64MB, even can change, maybe not good to
use in large size?
Yeah that's just a figleaf so we don't have to bother about the accounting
issue.
3. The migration and adaptation of the driver is also a challenge, and
currently, we are unable to control it.
Why does a udmabuf fd not work instead of any other dmabuf fd? That
shouldn't matter for the consuming driver ...
Perhaps implementing `copy_file_range` would be more suitable for us.
See my other mail, fundamentally these all rely on struct page being
present, and dma-buf doesn't give you that. Which means you need to go
below the dma-buf abstraction. And udmabuf is pretty much the thing for
that, because it wraps normal struct page memory into a dmabuf.
And copy_file_range on the underlying memfd might already work, I haven't
checked though.