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

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Message-ID: <aD8Gi9ShWDEYqWjB@infradead.org>
Date: Tue, 3 Jun 2025 07:28:27 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Christian König <christian.koenig@....com>
Cc: Christoph Hellwig <hch@...radead.org>, wangtao <tao.wangtao@...or.com>,
	sumit.semwal@...aro.org, kraxel@...hat.com,
	vivek.kasireddy@...el.com, viro@...iv.linux.org.uk,
	brauner@...nel.org, hughd@...gle.com, akpm@...ux-foundation.org,
	amir73il@...il.com, benjamin.gaignard@...labora.com,
	Brian.Starkey@....com, jstultz@...gle.com, tjmercier@...gle.com,
	jack@...e.cz, baolin.wang@...ux.alibaba.com,
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	bintian.wang@...or.com, yipengxiang@...or.com, liulu.liu@...or.com,
	feng.han@...or.com
Subject: Re: [PATCH v4 0/4] Implement dmabuf direct I/O via copy_file_range

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.