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

["Christian König" <christian.koenig@amd.com>]

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).

We have seen something similar with customers systems which we couldn't explain so far.

>> 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.

Perfect, thanks for that info.

Regards,
Christian.