Theodore Tso wrote:
> 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:
I don't know for sure why it should be invalid; I think a preallocated
block, since it has an *actual* *block* *allocated* after all, should
have that block number. But if it's going to be fake, let's not use a
"real" one like the superblock location...
A real block nr does eventually get assigned when we do getblock with
create=1 AFAICT.
>> @@ -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()?
For that matter, why do we call unmap_underlying_metadata at all, ever?
> 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?
I have this in F11 now, but it's giving me the heebie-jeebies still. At
least it's confined to preallocation (one of the great new ext4 features
I've been promoting recently... :)
-Eric
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