linux-kernel - Re: [PATCH v4 0/4] Implement dmabuf direct I/O via copy_file

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Message-ID: <aEg1BZj-HzbgWKsx@infradead.org>
Date: Tue, 10 Jun 2025 06:37:09 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Christian König <christian.koenig@....com>
Cc: wangtao <tao.wangtao@...or.com>, Christoph Hellwig <hch@...radead.org>,
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Subject: Re: [PATCH v4 0/4] Implement dmabuf direct I/O via copy_file_range

On Tue, Jun 10, 2025 at 12:52:18PM +0200, Christian König wrote:
> >> 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.
> 
> Work on that is already underway for some time.
> 
> Most GPU drivers already do sg_table -> DMA array conversion, I need
> to push on the remaining to clean up.

Do you have a pointer?

> >> 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).
> 
> Even on a low end CPU walking the page tables and grabbing references
> shouldn't be that much of an overhead.

Yes.

> 
> There must be some reason why you see so much CPU overhead. E.g.
> compound pages are broken up or similar which should not happen in
> the first place.

pin_user_pages outputs an array of PAGE_SIZE (modulo offset and shorter
last length) array strut pages unfortunately.  The block direct I/O
code has grown code to reassemble folios from them fairly recently
which did speed up some workloads.

Is this test using the block device or iomap direct I/O code?  What
kernel version is it run on?