linux-ext4 - Re: [PATCH v3 3/6] ext4: cache extent hole in extent status tree for ext4_da_map

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Date:	Mon, 1 Sep 2014 22:43:50 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Zheng Liu <gnehzuil.liu@...il.com>
Cc:	linux-ext4@...r.kernel.org,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Jan Kara <jack@...e.cz>, Zheng Liu <wenqing.lz@...bao.com>
Subject: Re: [PATCH v3 3/6] ext4: cache extent hole in extent status tree for
 ext4_da_map_blocks()

On Thu, Aug 07, 2014 at 11:35:50AM +0800, Zheng Liu wrote:
> From: Zheng Liu <wenqing.lz@...bao.com>
> 
> Currently extent status tree doesn't cache extent hole when a write
> looks up in extent tree to make sure whether a block has been allocated
> or not.  In this case, we don't put extent hole in extent cache because
> later this extent might be removed and a new delayed extent might be
> added back.  But it will cause a defect when we do a lot of writes.
> If we don't put extent hole in extent cache, the following writes also
> need to access extent tree to look at whether or not a block has been
> allocated.  It brings a cache miss.  This commit fixes this defect.
> Meanwhile, if an inode has no any extent, this extent hole also will
> be cached.

Hi Zheng,

I thought the reason why we have the EXT4_GET_BLOCKS_NO_PUT_HOLE flag
is because in ext4_da_map_blocks(), if there is a hole, we will be
immediately following it up with a call to ext4_es_insert_extent() to
fill in the hole with the EXTENT_STATUS_DELAYED flag.  The only time
we don't is if we run into an ENOSPC error.

Am I missing something?

						- Ted
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