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Date:   Thu, 21 Jan 2021 15:47:44 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Mikulas Patocka <mpatocka@redhat.com>
Cc:     Jan Kara <jack@suse.cz>, Dave Chinner <david@fromorbit.com>,
        Zhongwei Cai <sunrise_l@sjtu.edu.cn>,
        Theodore Ts'o <tytso@mit.edu>,
        David Laight <David.Laight@aculab.com>,
        Mingkai Dong <mingkaidong@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Eric Sandeen <esandeen@redhat.com>,
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        Wang Jianchao <jianchao.wan9@gmail.com>,
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        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-nvdimm <linux-nvdimm@lists.01.org>
Subject: Re: Expense of read_iter
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On Wed, Jan 20, 2021 at 10:12:01AM -0500, Mikulas Patocka wrote:
> Do you have some idea how to optimize the generic code that calls 
> ->read_iter?

Yes.

> It might be better to maintain an f_iocb_flags in the
> struct file and just copy that unconditionally.  We'd need to remember
> to update it in fcntl(F_SETFL), but I think that's the only place.

Want to give that a try?