linux-ext4 - Re: Delayed allocation and page_lock vs transaction start ordering

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Message-Id: <1208282932.3636.9.camel@localhost.localdomain>
Date:	Tue, 15 Apr 2008 11:08:52 -0700
From:	Mingming Cao <cmm@...ibm.com>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-ext4@...r.kernel.org, sandeen@...hat.com
Subject: Re: Delayed allocation and page_lock vs transaction start ordering

On Tue, 2008-04-15 at 18:14 +0200, Jan Kara wrote:
>   Hi,
> 
>   I've ported my patch inversing locking ordering of page_lock and
> transaction start to ext4 (on top of ext4 patch queue). Everything except
> delayed allocation is converted (the patch is below for interested
> readers). The question is how to proceed with delayed allocation. Its
> current implementation in VFS is designed to work well with the old
> ordering (page lock first, then start a transaction). We could bend it to
> work with the new locking ordering but I really see no point since ext4 is
> the only user. 

I think the plan is port the changes to ext2/3/JFS and support delayed
allocation on those filesystems. 

> Also XFS has AFAIK ordering first start transaction, then
> lock pages so if we should ever merge delayed alloc implementations the new
> ordering would make it easier.
>   So what do people think here? Do you agree with reimplementing current
> mpage_da_... functions?

It worth a try, but I could not see how to bend delayed allocation to
work the new ordering:( With delayed allocation Ext4 gets into
writepage() directly with page locked, but we need to start transaction
to do block allocation...:(

I guess this reserve locking ordering allows support writepages() for
ext3/4? What other the benefits?

Regards,
Mingming

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