Re: ext4_fallocate

[Zheng Liu <gnehzuil.liu@gmail.com>]

Hi Zach,

Thanks for reviewing this patch.

On Tue, Jul 03, 2012 at 10:57:35AM -0700, Zach Brown wrote:
> 
> >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.

Oh, actually now the default value is set to 1MB in this patch.  But I
think maybe other users want to change this value.  So I add an interface
in sysfs to adjust dynaimically.  Certainly it is convenient for me to do
the above tests. :-)

> 
> >+	/* 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?

I don't think 's_extent_zeroout_len' is updated frequently by user, but
using an on-stack copy quite can avoid the risk.  I will fix it if most
of all think that this patch is useful and can be applied.

Regards,
Zheng