linux-ext4 - Re: [PATCH 0/2] Add inode checksum support to ext4

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Message-ID: <20110801050435.GC5716@noexit.corp.google.com>
Date:	Sun, 31 Jul 2011 22:04:35 -0700
From:	Joel Becker <jlbec@...lplan.org>
To:	Coly Li <colyli@...il.com>
Cc:	Andreas Dilger <adilger.kernel@...ger.ca>,
	"Darrick J. Wong" <djwong@...ibm.com>,
	Andreas Dilger <aedilger@...il.com>,
	Mingming Cao <cmm@...ibm.com>, Theodore Ts'o <tytso@....edu>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] Add inode checksum support to ext4

On Sun, Jul 31, 2011 at 09:57:11PM -0700, Joel Becker wrote:
> On Mon, Aug 01, 2011 at 07:52:41AM +0800, Coly Li wrote:
> > On 2011年07月31日 15:08, Joel Becker Wrote:
> > > On Sat, Jul 30, 2011 at 03:25:32PM +0800, Coly Li wrote:
> > >> And in non-journal mode, there is not copy of any meta data block in jbd2, we need to be
> > >> more careful in check summing, e.g. inode/block bitmap blocks...
> > > 
> > > 	Sure, but you could use a trigger in journaled mode and then do
> > > the checksums directly in the __ext4_handle_journal_dirty_*() functions
> > > in non-journaled mode.  Sure, it would be a little more CPU time, but
> > > the user picked "checksums + no journal" at mkfs time.
> > > 
> > 
> > Yes, my idea was similar to you.
> > One thing not clear to me is, in non-journal mode, how to make the page of bitmap block being stable. Because bits
> > setting in Ext4 bitmap is non-locking, it might be possible that new bit setting after check sum is calculated.
> 
> 	Every place that changes the bits will eventually call
> ext4_journal_dirty(), which recalculates the checksum.  So there's no
> danger of a set-bit-after-last-checksum.  But you will have to lock
> around the checksum calculation in non-journaling mode.  JBD2 handles it
> for journaling mode.

	Wait, bitsetting in ext4 can't be non-locking.  Or are they
crazily stomping on memory?  I sure see an assert_spin_locked() in
mb_mark_used().

Joel

-- 

"Well-timed silence hath more eloquence than speech."  
         - Martin Fraquhar Tupper

			http://www.jlbec.org/
			jlbec@...lplan.org
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