linux-ext4 - Re: Discussion: is it time to remove dioread

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Message-Id: <20200108104520.3BC4A4203F@d06av24.portsmouth.uk.ibm.com>
Date:   Wed, 8 Jan 2020 16:15:13 +0530
From:   Ritesh Harjani <riteshh@...ux.ibm.com>
To:     Jan Kara <jack@...e.cz>, "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        joseph.qi@...ux.alibaba.com, Liu Bo <bo.liu@...ux.alibaba.com>
Subject: Re: Discussion: is it time to remove dioread_nolock?

Hello Ted/Jan,

On 1/7/20 10:52 PM, Jan Kara wrote:
> On Tue 07-01-20 12:11:09, Theodore Y. Ts'o wrote:
>> Hmm..... There's actually an even more radical option we could use,
>> given that Ritesh has made dioread_nolock work on block sizes < page
>> size.  We could make dioread_nolock the default, until we can revamp
>> ext4_writepages() to write the data blocks first....

Agreed. I guess it should be a straight forward change to make.
It should be just removing test_opt(inode->i_sb, DIOREAD_NOLOCK) 
condition from ext4_should_dioread_nolock().

> 
> Yes, that's a good point. And I'm not opposed to that if it makes the life
> simpler. But I'd like to see some performance numbers showing how much is
> writeback using unwritten extents slower so that we don't introduce too big
> regression with this...
> 

Yes, let me try to get some performance numbers with dioread_nolock as
the default option for buffered write on my setup.

AFAIU this should also fix the stale data exposure race between DIO
read and ext4_page_mkwrite, since we will by default be using unwritten
extents.
Currently I am testing the patch to fix this race which is based on our 
previous discussion. Will anyway post that. After that I can also 
collect the performance numbers for this suggested option (to make
dioread_nolock as default)

-ritesh