On Wed, Apr 29, 2009 at 10:17:20AM +0530, Aneesh Kumar K.V wrote:
> We need to mark the buffer_head mapping prealloc space
> as new during write_begin. Otherwise we don't zero out the
> page cache content properly for a partial write. This will
> cause file corruption with preallocation.
>
> Also use block number -1 as the fake block number so that
> unmap_underlying_metadata doesn't drop wrong buffer_head
The buffer_head code is starting to scare me more and more.
I'm looking at this code again and I can't figure out why it's safe
(or why we would need to) put in an invalid number into
bh_result->b_blocknr:
> @@ -2323,6 +2323,16 @@ static int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
> set_buffer_delay(bh_result);
> } else if (ret > 0) {
> bh_result->b_size = (ret << inode->i_blkbits);
> + /*
> + * With sub-block writes into unwritten extents
> + * we also need to mark the buffer as new so that
> + * the unwritten parts of the buffer gets correctly zeroed.
> + */
> + if (buffer_unwritten(bh_result)) {
> + bh_result->b_bdev = inode->i_sb->s_bdev;
> + set_buffer_new(bh_result);
> + bh_result->b_blocknr = -1;
Why do we need to avoid calling unmap_underlying_metadata()?
And after the buffer is zero'ed out, it leaves b_blocknr in a
buffer_head attached to the page at an invalid block number. Doesn't
that get us in trouble later on?
I see that this line is removed later on in the for-2.6.31 patch "Mark
the unwritten buffer_head as mapped during write_begin". But is it
safe for 2.6.30?
- Ted
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