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From: =?ISO-8859-15?Q?Luk=E1=A8_Czerner?= <lczerner@redhat.com>
Subject: Re: [PATCH] ext4: fix overhead calculation in bigalloc filesystem
 (Re: ... )
Date: Thu, 21 Feb 2013 13:50:03 +0100 (CET)
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Cc: Zheng Liu <gnehzuil.liu@gmail.com>, linux-ext4@vger.kernel.org,
	"Theodore Ts'o" <tytso@mit.edu>
To: =?ISO-8859-15?Q?Luk=E1=A8_Czerner?= <lczerner@redhat.com>
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On Thu, 21 Feb 2013, LukᨠCzerner wrote:

..snip...

> 
> Hi Zheng,
> 
> thanks for the review. I know about the other issues and I'm trying
> to resolve those as well. Right now I have a patch which includes
> the changes ext4_calculate_overhead() you've described below and more,
> but even with this I still see some problems remaining.
> 
> Hopefully will send another patch soon.
> 
> Thanks!
> -Lukas
> 
> > 
> > Subject: [PATCH] ext4: fix overhead calculation in bigalloc filesystem
> > 
> > From: Zheng Liu <wenqing.lz@taobao.com>
> > 
> > ext4_calculate_overhead() should compute the overhead and stash it in
> > sbi->s_overhead.  But we miss use EXT4_B2C() to calculate the number of
> > clusters before first_data_block and the number of journal blocks.  This
> > commit use EXT4_NUM_B2C() instead of EXT4_B2C() to calculate the
> > overhead.
> > 
> > Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
> > Cc: "Theodore Ts'o" <tytso@mit.edu>
> > ---
> >  fs/ext4/super.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> > index 3d4fb81..6165558 100644
> > --- a/fs/ext4/super.c
> > +++ b/fs/ext4/super.c
> > @@ -3219,7 +3219,7 @@ int ext4_calculate_overhead(struct super_block *sb)
> >  	/*
> >  	 * All of the blocks before first_data_block are overhead
> >  	 */
> > -	overhead = EXT4_B2C(sbi, le32_to_cpu(es->s_first_data_block));
> > +	overhead = EXT4_NUM_B2C(sbi, le32_to_cpu(es->s_first_data_block));

...except this. I do not think this is right because we do not skip
the first cluster right ? We're still using it, but we can never use
the block before es->s_first_data_block. Please correct me if I am
wrong.


> >  
> >  	/*
> >  	 * Add the overhead found in each block group
> > @@ -3235,7 +3235,7 @@ int ext4_calculate_overhead(struct super_block *sb)
> >  	}
> >  	/* Add the journal blocks as well */
> >  	if (sbi->s_journal)
> > -		overhead += EXT4_B2C(sbi, sbi->s_journal->j_maxlen);
> > +		overhead += EXT4_NUM_B2C(sbi, sbi->s_journal->j_maxlen);

This I already have in my patch I'm testing right now. And as I said
there are other places where we misuse EXT4_B2C().

-Lukas

> >  
> >  	sbi->s_overhead = overhead;
> >  	smp_wmb();
> > 
> 

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