On Fri, Jan 14, 2011 at 11:29:00AM +1100, Dave Chinner wrote: | This seems to be incorrect to me - a "wasdelay" extent has not yet | been initialised - there's data in memory, but there is nothing on | disk and we may not write it for some time. If we crash after this | transaction is written but before any data is written, we expose | stale data. | | Not only that, it allocates the _entire_ delalloc extent that spans | the preallocation range, even when the preallocation range is only 1 | block and the delalloc extent covers gigabytes. hence we actually | expose a much greater range of the file to stale data exposure | during a crash than just eh preallocated range. Not good. | | Secondly, I think we have the same expose-the-entire-delalloc-extent | -to-stale-data-exposure problem in ->writepage. This onnne, however, | is due to using BMAPI_ENTIRE to allocate the entire delalloc extent | the first time any part of it is written to. Even if we are only | writing a single page (i.e. wbc->nr_to_write = 1) and the delalloc | extent covers gigabytes. So, same problem when we crash. | | Finally, I think the extsize based problem exposed by test 229 is a | also a result of allocating space we have no pages covering in the | page cache (triggered by BMAPI_ENTIRE allocation) so the allocated | space is never zeroed and hence exposes stale data. There used to be an XFS_BMAPI_EXACT flag that wasn't ever used. What would be the effects of re-creating this flag and using it in writepage to prevent the expose-the-entire-delalloc-extent-to-stale-data-exposure problem? This wouldn't solve the exposure of stale data for a crash that occurs after the extent conversion but before the data is written out. The quantity of data exposed is potentially much smaller however. Also, I'm not saying using XFS_BMAPI_EXACT is feasable. I have a very minimal understanding of the writepage code path. -- Geoffrey Wehrman 651-683-5496 gwehrman@xxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
