Re: [PATCH 2/3] ext4: Clear the unwritten buffer_head flag properly

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Mon, May 11, 2009 at 11:08:56PM -0400, Theodore Tso wrote:
> On Thu, May 07, 2009 at 04:09:29PM +0530, Aneesh Kumar K.V wrote:
> > ext4_get_blocks_wrap does a block lookup requesting to
> > allocate new blocks. A lookup of blocks in prealloc area
> > result in setting the unwritten flag in buffer_head. So
> > a write to an unwritten extent will cause the buffer_head
> > to have unwritten and mapped flag set. Clear hte unwritten
> > buffer_head flag before requesting to allocate blocks.
> > 
> > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> 
> I've rewritten the commit changelog to this, which I believe more
> accurately describes the patch.  Comments, please?
> 
> ext4: Clear the unwritten buffer_head flag after the extent is initialized
> 
> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
> 
> The BH_Unwritten flag indicates that the buffer is allocated on disk
> but has not been written; that is, the disk was part of a persistent
> preallocation area.  That flag should only be set when a get_blocks()
> function is looking up a inode's logical to physical block mapping.
> 
> When ext4_get_blocks_wrap() is called with create=1, the uninitialized
> extent is converted into an initialized one, so the BH_Unwritten flag
> is no longer appropriate.  Hence, we need to make sure the
> BH_Unwritten is not left set, to avoid the ensuing confusion and
> hilarty.
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
> 

I think it is good. But one thing missing in the commit message is,
what happens if we do a write to prealloc space. Since a
get_block(create = 1) is now split into __get_block(create = 0 )  and
__get_block(create = 1). That would mean if we pass a buffer head with
BH_Unwritten cleared we will have


1) buffer_head as BH_Unwritten cleared.

2) __get_block(create = 0 ) -> Since it is prealloc space we will have
BH_Unwritten set .

3) __get_block(create = 1) -> get the blocks out of prealloc space.
and retun with BH_Mapped set. 

That would imply we have BH_Unwritten and BH_Mapped set in the above
case which is wrong. So we need a BH_Unwritten clear between (2) and
(3). The patch does the same. May be we need to capture it in commit
message.

-aneesh