linux-ext4 - Re: ext4

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Message-ID: <4FF3328F.3090301@zabbo.net>
Date:	Tue, 03 Jul 2012 10:57:35 -0700
From:	Zach Brown <zab@...bo.net>
To:	Ric Wheeler <rwheeler@...hat.com>, Jan Kara <jack@...e.cz>,
	Eric Sandeen <sandeen@...hat.com>,
	Theodore Ts'o <tytso@....edu>, Fredrick <fjohnber@...o.com>,
	linux-ext4@...r.kernel.org, Andreas Dilger <adilger@...ger.ca>,
	wenqing.lz@...bao.com
Subject: Re: ext4_fallocate


> workload frequently does a uninitialized extent conversion.  Thus, now
> we set it to 256 (1MB chunk), and put it into super block in order to
> adjust it dynamically in sysfs.

It's a bit of a tangent, but this caught my eye.

> +	/* If extent has less than 2*s_extent_zeroout_len zerout directly */
> +	if (ee_len<= 2*sbi->s_extent_zeroout_len&&
>   	(EXT4_EXT_MAY_ZEROOUT&  split_flag)) {

> -		if (allocated<= EXT4_EXT_ZERO_LEN&&
> +		if (allocated<= sbi->s_extent_zeroout_len&&
>   		(EXT4_EXT_MAY_ZEROOUT&  split_flag)) {

>   		} else if ((map->m_lblk - ee_block + map->m_len<
> -			   EXT4_EXT_ZERO_LEN)&&
> +			   sbi->s_extent_zeroout_len)&&

I'd be worried about having to verify that nothing bad happened if these
sbi s_extent_zeroout_len references could see different values if they
raced with a sysfs update.  Can they do that?

Maybe avoid the risk all together and have an on-stack copy that's only
referenced once at the start?

- z
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