linux-ext4 - Re: [PATCH] ext4: calculate journal credits correctly

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Date:	Wed, 12 Nov 2008 16:03:42 -0800
From:	Mingming Cao <cmm@...ibm.com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Aneesh Kumar <aneesh.kumar@...il.com>
Subject: Re: [PATCH] ext4: calculate journal credits correctly


在 2008-11-06四的 14:51 -0500，Theodore Ts'o写道：
> This fixes a 2.6.27 regression which was introduced in commit a02908f1.
> 
> We weren't passing the chunk parameter down to the two subections,
> ext4_indirect_trans_blocks() and ext4_ext_index_trans_blocks(), with
> the result that massively overestimate the amount of credits needed by
> ext4_da_writepages, especially in the non-extents case.  This causes
> failures especially on /boot partitions, which tend to be small and
> non-extent using since GRUB doesn't handle extents.
> 
> Thanks to Joseph Fannin for reporting this bug.
> 
Thanks for fixing this up!

However, looking at the code path, I wasn't sure if we got the delalloc
credits right.  In ext4_da_writepages()->mpage_da_writepages(),  the
credits is calculated based on the assumption that mpage_da_writepages()
doing* one* single chunk of contigugous allocation? So only one single
extent credit is reserved (here you see the "chunk" flag passed from the
ext4_da_writepages)

__mpage_da_writepage() does do single chunk of block allocation at a
time, but mpage_da_writepages()->write_cache_pages() will loop the page
vectors and probably will calling writepage->__mpage_da_writepage->
mpage_da_map_blocks() multiple times?  Am I  missing anything?


> Signed-off-by: "Theodore Ts'o" <tytso@....edu>
> ---
>  fs/ext4/inode.c |    5 +++--
>  1 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 8dbf695..5a130b5 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -4580,9 +4580,10 @@ static int ext4_indirect_trans_blocks(struct inode *inode, int nrblocks,
>  static int ext4_index_trans_blocks(struct inode *inode, int nrblocks, int chunk)
>  {
>  	if (!(EXT4_I(inode)->i_flags & EXT4_EXTENTS_FL))
> -		return ext4_indirect_trans_blocks(inode, nrblocks, 0);
> -	return ext4_ext_index_trans_blocks(inode, nrblocks, 0);
> +		return ext4_indirect_trans_blocks(inode, nrblocks, chunk);
> +	return ext4_ext_index_trans_blocks(inode, nrblocks, chunk);
>  }
> +
>  /*
>   * Account for index blocks, block groups bitmaps and block group
>   * descriptor blocks if modify datablocks and index blocks

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