Theodore Tso wrote:
> On Tue, Apr 28, 2009 at 03:01:45PM +0530, Aneesh Kumar K.V wrote:
>> Looking at the source again i guess setting just b_dev is not enough.
>> unmap_underlying_metadata looks at the mapping block number, which we
>> don't have in case on unwritten buffer_head. How about the below patch ?
>> It involve vfs changes. But i guess it is correct with respect to the
>> meaning of BH_New (Disk mapping was newly created by get_block). I guess
>> BH_New implies BH_Mapped.
>
> Argh. So we have multiple problems going on here. One is the
> original problem, namely that of a partial write into an preallocated
> block can leave garbage behind in that unitialized block.
>
> The other problem seems to be in the case of a delayed allocation
> write, where we return a buffer_head which is marked new, and this
> causes block_prepare_write() to call unmap_underlying_metadata(dev, 0).
>
> In theory this could cause problems if we try installing a new
> bootloader in the filesystem's boot block while there's a delayed
> writes happening in the background, since we could end up discarding
> the write to the boot sector. We've lived with this for quite a wihle
> though.
>
> My concern with making the fs/buffer.c changes is that we need to make
> sure it doesn't break any of the other filesystems, so that's going to
> make it hard to try to slip this with 2.6.30-rc4 nearly upon us.
> (Silly question; why doesn't XFS get caught by this?)
I'm not sure offhand. All xfs does is this in the get_block path:
* 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 (create &&
((!buffer_mapped(bh_result) && !buffer_uptodate(bh_result)) ||
(offset >= i_size_read(inode)) ||
(iomap.iomap_flags & (IOMAP_NEW|IOMAP_UNWRITTEN))))
set_buffer_new(bh_result);
so it returns with BH_New as well.
> So the question is do we try to fix both bugs with one patch, and very
> likely have to wait until 2.6.31 before the patch is incorporated? Or
> do we fix the second bug using an ext4-only fix, with the knowledge
> that post 2.6.30, we'll need undo most of it and fix it properly with
> a change that involves fs/buffer.c?
I have the sense that this might need a bit more digging around, and I
finally got stuff out of the way to do so :)
-Eric
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