Re: [PATCH] ext4: Adjust trim start with first_data_block.

[Tao Ma <tm@tao.ma>]

Hi Ted,
On 02/04/2011 01:40 PM, Ted Ts'o wrote:
> On Fri, Jan 21, 2011 at 10:05:07PM +0800, Tao Ma wrote:
>    
>> From: Tao Ma<boyu.mt@taobao.com>
>>
>> As we have make the consense in the e-mail[1], the trim start should
>> be added with first_data_block. So this patch fulfill it and remove
>> the check for start<  first_data_block.
>>      
>    
>> [1] http://www.spinics.net/lists/linux-ext4/msg22737.html
>>
>>      
> Sorry, I was away at Linux.conf.au and didn't have a chance to respond
> this.
>    
np.
> Fundamentally I think we need to understand what the arguments to the
> trim ioctl is supposed to *mean*.  Are they supposed to be physical
> block numbers, or some thing else?  If we just bump everything by the
> first_data_block, (which is non-zero only for the 1k case), that will
> screw up the length argument, because the userspace program isn't
> going to know that (a) the file system is using 1k blocks, and (b) on
> ext2/3/4 that means that first_data_block is 1.
>    
fair enough.
> So instead of saying that the arguments to trim mean "the data blocks"
> --- which is a concept that doesn't really have any meaning to the
> caller of the trim ioctl, without forcing it to know a lot more about
> the physical layout of filesystems, I think the argument to trim
> should be the physical block numbers.
>
> And just as we don't trim blocks that contain data that should be
> saved, we should just simply not trim block #0 (the boot block) and
> and block #1 (the superblock) on 1k block filesystems, and not trim
> block #0 (the boot block as well as the superblock) on 4k block
> filesystems.  But we shouldn't be doing this by taking block number
> passed to trim and just adding first_data_block to it.
>    
I am open to any suggestion as:
1) you are the original author of ext2/3/4. ;)
2) I am not the original author of trim in ext3/4 and this patch is 
suggested by Lucas. So Lucas, do you agree with it?
> I know a patch to do that has already merged into ext3, but with the
> indulgence of the folks on linux-ext4, could we reopen this question?
>    
Never mind.
So if you has the objection to this patch, please consider merging 
another patch.
It fix an underflow problem and treat 'start' as the first physical 
block number.
http://marc.info/?l=linux-ext4&m=129543034911496&w=2

As for ext3, I will contact Jan for the update if you decide to abandon 
this patch and accept the patch for 'underflow'.

Regards,
Tao