linux-ext4 - Re: JBD commmit : Get list of all 'to be written' buffers in advance

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Date:	Wed, 18 May 2011 14:38:54 +0200
From:	Jan Kara <jack@...e.cz>
To:	niraj kulkarni <kulkarniniraj14@...il.com>
Cc:	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: JBD commmit : Get list of all 'to be written' buffers in
 advance

On Wed 18-05-11 13:38:47, niraj kulkarni wrote:
> Thanks for the reply. That solves most of the problem.
> 
> So now all I need is to gather information of all submitted buffers in
> current transactions, and pass it through ioctl to lower layer.
> What I am currently doing is adding buffer  to a list instead of submitting
> them straightaway, and side by side constructing an array holding these
> buffer numbers. So i submit this buffer number array through ioctl, and then
> actually submit buffer.
> 
> This needs a few kmallocs while I am holding buffer_lock() on few buffers.
> So is it potentially deadlocking situation?
  It shouldn't be if you use proper gfp mask - like GFP_NOFS should be OK.

									Honza
> On Tue, May 17, 2011 at 11:39 PM, Jan Kara <jack@...e.cz> wrote:
> 
> >  Hello,
> >
> > On Thu 12-05-11 19:52:21, Niraj Kulkarni wrote:
> > >          I am trying to implement a concept called Transactional
> > > Flash (
> > http://www.usenix.org/event/osdi08/tech/full_papers/prabhakaran/prabhakaran_html/
> > )
> > >
> > >          in which all writes are flushed to disk (in this case
> > > Flash) as a circular linked list of pages. No journaling mechanisms
> > > are used.
> > >           For that I am trying to get list of all buffers to be
> > > written (metadata or data) before actually writing them. I thought
> > > of 2 approaches :
> > >
> > > 1. Get conservative list of all buffers before flushing any of them
> > > (before commit phase 2) in journal_commit_transaction function.
> > >      Problem : Metadata writes use temporary buffers which are
> > > allocated just before write.
> >   Well, if I understand the transactional flash concept right, what you
> > should do is to replace code committing metadata to the journal by
> > something which writes metadata directly to final location. That should be
> > relatively easy modification of loops in journal_commit_transaction(). BTW,
> > you might want to use ext4/jbd2 instead of ext3/jbd as a base for your
> > experiments since ext3 is basically deprecated.
> >
> > > 2. Instead of submitting buffers straightaway, collect all of them (
> > > data + metadata ) and flush them in end.
> > >      Problem :  i.   Possibility of blocking after data buffers are
> > > collected but before metadata buffer collection.
> > >                            Since data buffers are locked, this may
> > > lead to deadlock. (I am not sure if it is spinlock)
> > >
> > >                       ii.   Not able to differentiate between
> > > metadata buffers and Descriptor buffers.
> >
> >                                                                 Honza
> > --
> > Jan Kara <jack@...e.cz>
> > SUSE Labs, CR
> >
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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