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@xxxxxxxxxxxxxxxxxx>
> ---
> fs/ext4/inode.c | 7 +++++++
> 1 files changed, 7 insertions(+), 0 deletions(-)
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index c3cd00f..f6d7e9b 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -1149,6 +1149,7 @@ int ext4_get_blocks_wrap(handle_t *handle, struct inode *inode, sector_t block,
> int retval;
>
> clear_buffer_mapped(bh);
> + clear_buffer_unwritten(bh);
>
> /*
> * Try to see if we can get the block without requesting
> @@ -1179,6 +1180,12 @@ int ext4_get_blocks_wrap(handle_t *handle, struct inode *inode, sector_t block,
> return retval;
>
> /*
> + * The above get_blocks can cause the buffer to be
> + * marked unwritten. So clear the same.
> + */
> + clear_buffer_unwritten(bh);
hm, thinking out loud here.
ext4_ext_get_blocks() will only set unwritten if (!create) ... but then
ext4_get_blocks_wrap() calls ext4_ext_get_blocks() !create as an
argument no matter what, the first time, for an initial lookup.
But if ext4_get_blocks_wrap() was called with !create, then we return
regardless, so ok - by the time you get to the above hunk, we -are- in
create mode, we're planning to write it ... so I guess clearing the
unwritten state makes sense here.
But is this too late, because it's after this?
/*
* Returns if the blocks have already allocated
*
* Note that if blocks have been preallocated
* ext4_ext_get_block() returns th create = 0
* with buffer head unmapped.
*/
if (retval > 0 && buffer_mapped(bh))
return retval;
I guess not; ext4_ext_get_blocks() won't map the buffer if it's found to
be preallocated/unwritten because it was called with !create. If we're
going on to write it, we want to clear unwritten.
So I guess this looks right, although I can't help but think that in
general, the buffer_head state management is really getting to be a
hard-to-follow mess...
-Eric
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