linux-ext4 - Re: semi-stable page writes

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Date:	Tue, 30 Oct 2012 09:01:22 +1100
From:	Dave Chinner <david@...morbit.com>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	Theodore Ts'o <tytso@....edu>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: semi-stable page writes

On Fri, Oct 26, 2012 at 03:19:09AM -0700, Darrick J. Wong wrote:
> Hi everyone,
> 
> Are people still annoyed about writes taking unexpectedly long amounts of tme
> due to the stable page write patchset?  I'm guessing yes...

I haven't heard anyone except th elunatic fringe complain
recently...

> I'm close to posting a patchset that (a) gates the wait_on_page_writeback calls
> on a flag that you can set in the bdi to indicate that you need stable writes
> (which blk_integrity_register will set);

I'd prefer stable pages by default (e.g. btrfs needs it for sane
data crc calculations), with an option to turn it off.

> (b) (ab)uses a page flag bit (PG_slab)
> to indicate that a page is actually being sent out to disk hardware; and (c)

I don't think you can do that. You can send slab allocated memory to
disk (e.g. kmalloc()d memory) and XFS definitely does that for
sub-page sized metadata. I'm pretty sure that means the PG_slab
flag is not available for (ab)use in the IO path....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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