Re: [PATCH 0/2] Support direct I/O read and write for memory allocated by dmabuf

[Christian König]

Am 10.07.24 um 16:35 schrieb Lei Liu:
> 在 2024/7/10 22:14, Christian König 写道:
> > Am 10.07.24 um 15:57 schrieb Lei Liu:
> > > Use vm_insert_page to establish a mapping for the memory allocated
> > > by dmabuf, thus supporting direct I/O read and write; and fix the
> > > issue of incorrect memory statistics after mapping dmabuf memory.
> >
> > Well big NAK to that! Direct I/O is intentionally disabled on DMA-bufs.
>
> Hello! Could you explain why direct_io is disabled on DMABUF? Is there 
> any historical reason for this?

It's basically one of the most fundamental design decision of DMA-Buf. 
The attachment/map/fence model DMA-buf uses is not really compatible 
with direct I/O on the underlying pages.

> > We already discussed enforcing that in the DMA-buf framework and this 
> > patch probably means that we should really do that.
> >
> > Regards,
> > Christian.
>
> Thank you for your response. With the application of AI large model 
> edgeification, we urgently need support for direct_io on DMABUF to 
> read some very large files. Do you have any new solutions or plans for 
> this?

We have seen similar projects over the years and all of those turned out 
to be complete shipwrecks.

There is currently a patch set under discussion to give the network 
subsystem DMA-buf support. If you are interest in network direct I/O 
that could help.

Additional to that a lot of GPU drivers support userptr usages, e.g. to 
import malloced memory into the GPU driver. You can then also do direct 
I/O on that malloced memory and the kernel will enforce correct handling 
with the GPU driver through MMU notifiers.

But as far as I know a general DMA-buf based solution isn't possible.

Regards,
Christian.

> Regards,
> Lei Liu.
>
> > > Lei Liu (2):
> > >    mm: dmabuf_direct_io: Support direct_io for memory allocated by 
> > > dmabuf
> > >    mm: dmabuf_direct_io: Fix memory statistics error for dmabuf 
> > > allocated
> > >      memory with direct_io support
> > >
> > >   drivers/dma-buf/heaps/system_heap.c |  5 +++--
> > >   fs/proc/task_mmu.c                  |  8 +++++++-
> > >   include/linux/mm.h                  |  1 +
> > >   mm/memory.c                         | 15 ++++++++++-----
> > >   mm/rmap.c                           |  9 +++++----
> > >   5 files changed, 26 insertions(+), 12 deletions(-)