linux-ext4 - Re: [RFC PATCH] ext4: Return unwritten buffer head when trying to read from prealloc space.

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Date:	Fri, 21 Mar 2008 15:13:58 -0700
From:	Mingming Cao <cmm@...ibm.com>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	tytso@....edu, sandeen@...hat.com, Solofo.Ramangalahy@...l.net,
	dmonakhov@...nvz.org, linux-ext4@...r.kernel.org
Subject: Re: [RFC PATCH] ext4: Return unwritten buffer head when trying to
	read from prealloc space.

On Fri, 2008-03-21 at 11:48 +0530, Aneesh Kumar K.V wrote:
> ext4_ext_get_blocks  returns number of blocks allocated with buffer head
> unmapped for a read from prealloc space. This is needed so that delayed
> allocation doesn't do block reservation for prealloc space since the blocks
> are already resevred on disk. Fix ext4_ext_get_blocks to not return greater
> than max_blocks and also mark the buffer head unwritten. Some code path tries
> to read the block if buffer_head is not new and no uptodate. Marking the buffer
> head unwritten avoid this reading.
> 

Seems this patch fixes two bugs together, it would be nice to split to
two as they cause two different problems.

To fix the bh->b_size warning solof founds for delayed allocation, I
would say we need to fix it in ext4_get_blocks_handle() (for indirect
files) and ext4_ext_get_blocks()(for extent files) both, not allowing
returning b_size greater than what the caller asking for. Also, the
patch below seems only address the preallocation map case, we need to
fix in in general block lookup case.

> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
> ---
>  fs/ext4/extents.c |   14 +++++++++++++-
>  1 files changed, 13 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index d6ae40a..edd1bf2 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -2600,8 +2600,20 @@ int ext4_ext_get_blocks(handle_t *handle, struct inode *inode,
>  			}
>  			if (create == EXT4_CREATE_UNINITIALIZED_EXT)
>  				goto out;
> -			if (!create)
> +			if (!create) {
> +				/*
> +				 * We have blocks reserved already. We
> +				 * return allocated blocks so that delalloc
> +				 * won't do block reservation for us. But
> +				 * the buffer head will be unmapped so that
> +				 * a read from the block return 0
> +				 */
> +				if (allocated > max_blocks)
> +					allocated = max_blocks;
> +				/* mark the buffer unwritten */
> +				__set_bit(BH_Unwritten, &bh_result->b_state);
>  				goto out2;
> +			}
> 
>  			ret = ext4_ext_convert_to_initialized(handle, inode,
>  								path, iblock,

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