On Wed 29-04-09 09:08:05, Eric Sandeen wrote: > Jan Kara wrote: > >> Aneesh Kumar K.V wrote: > >>> On Tue, Apr 28, 2009 at 01:00:47PM -0400, Theodore Tso wrote: > >>>> On Tue, Apr 28, 2009 at 10:05:54PM +0530, Aneesh Kumar K.V wrote: > >> ... > >>>>>> 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). > >>>>> Not just that. On block allocation we are not calling > >>>>> unmap_underlying_metadata(dev, blocknumber) for delayed allocated > >>>>> blocks. That would imply file corruption. > >>>> I don't think I'm following you . If we write into block that was > >>>> delayed allocated. Are you saying we might get in trouble of the > >>>> delayed allocation block is mmap'ed in? > >>> We allocate blocks for delayed buffer during writepage. Now we need to > >>> make sure after getting the blocks we drop the old buffer_head mapping > >>> that we may have with this particular block attached to the block > >>> device. That is done by calling unmap_underlying_metadata. Now the > >>> current code doesn't call unmap_underlying_metadata for delayed > >>> allocated blocks. That would mean we can see corrupt files if old > >>> buffer_head mapping gets synced to disk AFTER we write the new > >>> buffer_head mapping. > >> > >> Talking w/ Aneesh on IRC, I don't see how we can have stray dirty > >> mappings lying around for this block device unless someone is writing > >> directly to the mounted block device, which I don't think is ever > >> considered safe ... > >> > >> I'm not quite sure what the call to __unmap_underlying_blocks() in > >> mpage_da_map_blocks() is for, I guess? > > For ext3 / ext4 I think we don't need unmap_underlying_blocks() since > > before we reallocate a block, we make sure that the transaction freeing > > the block is committed and clear all dirty bits from freed blocks. > > But for more careless filesystems, if they reallocate metadata block > > as a data block and don't clear the dirty bit in blockdev mapping, > > unmap_underlying_blocks() does it for them. > > That's what I thought - so I was wondering why we have specific calls to > this in ext4: > > mpage_da_map_blocks > __unmap_underlying_blocks > for (i = 0; i < blocks; i++) > unmap_underlying_metadata Hmm, OK. So maybe change it warn on dirty blockdev buffer and if the warning does not trigger we can believe that our theory is right ;). Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
