Re: [PATCH 1/2] dma-buf: heaps: DMA_HEAP_IOCTL_ALLOC_READ_FILE framework

[Daniel Vetter <daniel.vetter@ffwll.ch>]

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.

Cheers, Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch