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 02:07:20PM +0200, Christian König wrote:
> Am 16.07.24 um 11:31 schrieb Daniel Vetter:
> > 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 ...
> 
> Pinning (or rather taking another page reference) prevents writes from using
> direct I/O because writes try to find all references and make them read only
> so that nobody modifies the content while the write is done.

Where do you see that happen? That's counter to my understading of what
pin_user_page() does, which is what direct I/O uses ...

> As far as I know the same approach is used for NUMA migration and replacing
> small pages with big ones in THP. But for the read case here it should still
> work.

Yeah elevated refcount breaks migration, but that's entirely different
from the direct I/O use-case. Count me somewhat confused.
-Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch