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

[Christoph Hellwig]

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?

> My question is rather if it's ok to call f_op->write_iter() and 
> f_op->read_iter() with pages allocated by alloc_pages(), e.g.
> where drivers potentially ignore the page count and just re-use pages
> as they like?

read_iter and write_iter with ITER_BVEC just use the pages as source
and destination of the I/O.  They must not touch the refcounts or
do anything fancy with them.  Various places in the kernel rely on
that.