RE: [PATCH v4 0/4] Implement dmabuf direct I/O via copy_file_range

[wangtao <tao.wangtao@honor.com>]

> -----Original Message-----
> From: Christoph Hellwig <hch@infradead.org>
> Sent: Monday, June 9, 2025 12:35 PM
> To: Christian König <christian.koenig@amd.com>
> Cc: wangtao <tao.wangtao@honor.com>; Christoph Hellwig
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> Subject: Re: [PATCH v4 0/4] Implement dmabuf direct I/O via
> copy_file_range
> 
> On Fri, Jun 06, 2025 at 01:20:48PM +0200, Christian König wrote:
> > > dmabuf acts as a driver and shouldn't be handled by VFS, so I made
> > > dmabuf implement copy_file_range callbacks to support direct I/O
> > > zero-copy. I'm open to both approaches. What's the preference of VFS
> > > experts?
> >
> > That would probably be illegal. Using the sg_table in the DMA-buf
> > implementation turned out to be a mistake.
> 
> Two thing here that should not be directly conflated.  Using the sg_table was
> a huge mistake, and we should try to move dmabuf to switch that to a pure
I'm a bit confused: don't dmabuf importers need to traverse sg_table to
access folios or dma_addr/len? Do you mean restricting sg_table access
(e.g., only via iov_iter) or proposing alternative approaches?

> dma_addr_t/len array now that the new DMA API supporting that has been
> merged.  Is there any chance the dma-buf maintainers could start to kick this
> off?  I'm of course happy to assist.
> 
> But that notwithstanding, dma-buf is THE buffer sharing mechanism in the
> kernel, and we should promote it instead of reinventing it badly.
> And there is a use case for having a fully DMA mapped buffer in the block
> layer and I/O path, especially on systems with an IOMMU.
> So having an iov_iter backed by a dma-buf would be extremely helpful.
> That's mostly lib/iov_iter.c code, not VFS, though.
Are you suggesting adding an ITER_DMABUF type to iov_iter, or
implementing dmabuf-to-iov_bvec conversion within iov_iter?

> 
> > The question Christoph raised was rather why is your CPU so slow that
> > walking the page tables has a significant overhead compared to the
> > actual I/O?
> 
> Yes, that's really puzzling and should be addressed first.
With high CPU performance (e.g., 3GHz), GUP (get_user_pages) overhead
is relatively low (observed in 3GHz tests).
|    32x32MB Read 1024MB    |Creat-ms|Close-ms|  I/O-ms|I/O-MB/s| I/O%
|---------------------------|--------|--------|--------|--------|-----
| 1)        memfd direct R/W|      1 |    118 |    312 |   3448 | 100%
| 2)      u+memfd direct R/W|    196 |    123 |    295 |   3651 | 105%
| 3) u+memfd direct sendfile|    175 |    102 |    976 |   1100 |  31%
| 4)   u+memfd direct splice|    173 |    103 |    443 |   2428 |  70%
| 5)      udmabuf buffer R/W|    183 |    100 |    453 |   2375 |  68%
| 6)       dmabuf buffer R/W|     34 |      4 |    427 |   2519 |  73%
| 7)    udmabuf direct c_f_r|    200 |    102 |    278 |   3874 | 112%
| 8)     dmabuf direct c_f_r|     36 |      5 |    269 |   4002 | 116%

With lower CPU performance (e.g., 1GHz), GUP overhead becomes more
significant (as seen in 1GHz tests).
|    32x32MB Read 1024MB    |Creat-ms|Close-ms|  I/O-ms|I/O-MB/s| I/O%
|---------------------------|--------|--------|--------|--------|-----
| 1)        memfd direct R/W|      2 |    393 |    969 |   1109 | 100%
| 2)      u+memfd direct R/W|    592 |    424 |    570 |   1884 | 169%
| 3) u+memfd direct sendfile|    587 |    356 |   2229 |    481 |  43%
| 4)   u+memfd direct splice|    568 |    352 |    795 |   1350 | 121%
| 5)      udmabuf buffer R/W|    597 |    343 |   1238 |    867 |  78%
| 6)       dmabuf buffer R/W|     69 |     13 |   1128 |    952 |  85%
| 7)    udmabuf direct c_f_r|    595 |    345 |    372 |   2889 | 260%
| 8)     dmabuf direct c_f_r|     80 |     13 |    274 |   3929 | 354%

Regards,
Wangtao.