On Fri, Jul 30, 2010 at 08:45:16AM +1000, Dave Chinner wrote: > If we get two unaligned direct IO's to the same filesystem block > that is marked as a new allocation (i.e. buffer_new), then both IOs will > zero the portion of the block they are not writing data to. As a > result, when the IOs complete there will be a portion of the block > that contains zeros from the last IO to complete rather than the > data that should be there. Urgh. Yuck. > This is easily manifested by qemu using aio+dio with an unaligned > guest filesystem - every IO is unaligned and fileystem corruption is > encountered in the guest filesystem. xfstest 240 (from Eric Sandeen) > is also a simple reproducer. > > To avoid this problem, track unaligned IO that triggers sub-block zeroing and > check new incoming unaligned IO that require sub-block zeroing against that > list. If we get an overlap where the start and end of unaligned IOs hit the > same filesystem block, then we need to block the incoming IOs until the IO that > is zeroing the block completes. The blocked IO can then continue without > needing to do any zeroing and hence won't overwrite valid data with zeros. Urgh. Yuck. Could we perhaps handle this by making an IO instantiate a page cache page for partial writes, and forcing that portion of the IO through the page cache? The second IO would hit the same page and use the existing O_DIRECT vs page cache paths. -- Matthew Wilcox Intel Open Source Technology Centre "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step." -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
