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

[Christoph Hellwig]

On Tue, Jun 03, 2025 at 05:55:18PM +0200, Christian König wrote:
> On 6/3/25 16:28, Christoph Hellwig wrote:
> > On Tue, Jun 03, 2025 at 04:18:22PM +0200, Christian König wrote:
> >>> Does it matter compared to the I/O in this case?
> >>
> >> It unfortunately does, see the numbers on patch 3 and 4.
> > 
> > That's kinda weird.  Why does the page table lookup tage so much
> > time compared to normal I/O?
> 
> I have absolutely no idea. It's rather surprising for me as well.
> 
> The user seems to have a rather slow CPU paired with fast I/O, but it still looks rather fishy to me.
> 
> Additional to that allocating memory through memfd_create() is *much* slower on that box than through dma-buf-heaps (which basically just uses GFP and an array).

Can someone try to reproduce these results on a normal system
before we're building infrastructure based on these numbers?