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 -Eric -- 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
