linux-kernel - Re: Expense of read

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Date:   Mon, 11 Jan 2021 00:18:05 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Steven Whitehouse <swhiteho@...hat.com>,
        Eric Sandeen <esandeen@...hat.com>,
        Dave Chinner <dchinner@...hat.com>,
        Theodore Ts'o <tytso@....edu>,
        Wang Jianchao <jianchao.wan9@...il.com>,
        "Kani, Toshi" <toshi.kani@....com>,
        "Norton, Scott J" <scott.norton@....com>,
        "Tadakamadla, Rajesh" <rajesh.tadakamadla@....com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-nvdimm@...ts.01.org
Subject: Re: Expense of read_iter

On Sun, Jan 10, 2021 at 04:19:15PM -0500, Mikulas Patocka wrote:
> I put counters into vfs_read and vfs_readv.
> 
> After a fresh boot of the virtual machine, the counters show "13385 4". 
> After a kernel compilation they show "4475220 8".
> 
> So, the readv path is almost unused.
> 
> My reasoning was that we should optimize for the "read" path and glue the 
> "readv" path on the top of that. Currently, the kernel is doing the 
> opposite - optimizing for "readv" and glueing "read" on the top of it.

But it's not about optimising for read vs readv.  read_iter handles
a host of other cases, such as pread(), preadv(), AIO reads, splice,
and reads to in-kernel buffers.

Some device drivers abused read() vs readv() to actually return different
information, depending which you called.  That's why there's now a
prohibition against both.

So let's figure out how to make iter_read() perform well for sys_read().