linux-ext4 - Re: [PATCH] ext4: add fallocate mode blocking for debugging purposes

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Date:	Tue, 15 Apr 2014 11:15:41 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Lukáš Czerner <lczerner@...hat.com>,
	"Theodore Ts'o" <tytso@....edu>
CC:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Namjae Jeon <linkinjeon@...il.com>
Subject: Re: [PATCH] ext4: add fallocate mode blocking for debugging purposes

On 4/15/14, 11:02 AM, Lukáš Czerner wrote:
> On Sun, 13 Apr 2014, Theodore Ts'o wrote:
> 
>> Date: Sun, 13 Apr 2014 18:00:16 -0400
>> From: Theodore Ts'o <tytso@....edu>
>> To: Ext4 Developers List <linux-ext4@...r.kernel.org>
>> Cc: Namjae Jeon <linkinjeon@...il.com>
>> Subject: Re: [PATCH] ext4: add fallocate mode blocking for debugging purposes
>>
>> On Sun, Apr 13, 2014 at 04:21:58PM -0400, Theodore Ts'o wrote:
>>> If a particular fallocate mode is causing test failures, give the
>>> tester the ability to block a particular fallocate mode so that the
>>> use of a particular fallocate mode will be reported as not supported.
>>>
>>> For example, if the COLLAPSE_RANGE fallocate mode is causing test
>>> failures, this allows us to suppress it so we can more easily test the
>>> rest of the file system code.
>>
>> Hi Namjae,
>>
>> One of the reasons this patch set is that after Lukas added
>> COLLAPSE_RANGE support into fsx, we've started seeing a number of
>> failures which seem to be directly related to COLLAPSE_RANGE.
> 
> Ah, I did mentioned it when I added COLLAPSE_RANGE to the fsx and
> fsstress, but I forgot to cc you Namjae, sorry about that.
> 
> But about the patch. It seems a little bit weird to change kernel
> for this. The way I am doing it is by changing ltp/fsx.c and
> ltp.fsstress.c to disable the particular mode:

I tend to agree, better to fix the kernel than to add a knob to turn it
off.  And fsx changes can happen a lot quicker than kernel changes.  [1]

And if it's really unsafe, and you really want to add a knob, I'd at least
default it to off until it's non-corrupting, and add a message that
this tunable will go away as soon as it's stable, so you'll have no
qualms about quickly deprecating it...

-Eric

[1] it'd be nifty to make an env. var in xfstests which can globally
disable certain fsx operations across all tests which run fsx...
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