On Wed, Aug 04, 2010 at 01:37:18PM +1000, Dave Chinner wrote: > On Tue, Aug 03, 2010 at 05:11:18PM -0700, Mingming Cao wrote: > > On Fri, 2010-07-30 at 08:45 +1000, Dave Chinner wrote: > > > From: Dave Chinner <dchinner@xxxxxxxxxx> > > > > > > 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. > > > > > > 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. > > > > > > > This seems to address both two IOs are unaligned direct IO. If the first > > IO is aligned direct IO, then it is not tracked? > > > > I am also concerned about the aligned direct IO case... > > > > 1) first thread aio+dio+aligned write to a hole, there is no zero-out > > submitted from kernel. But the hole remains initialized before all IO > > complete and convert it from uninitialized extent to initialized. > > 2) second thread aio+dio+unalign write to the same hole, this time it is > > unaligned. since buffer is still new (not converted yet), the new > > incoming thread zero out port of data that first thread has written to > > That is clearly and unmistakably an application bug - it should not > be issuing concurrent, overlapping IO to the same block(s) > regardless of whether they are unaligned, aligned or a mixture of > both. By using direct IO, the application has assumed responsibility > for preventing data corruption due to overlapping IOs - they are > inherently racy and nothing in the dio code prevents that from > occurring. > > The bug I'm fixing is for *non-overlapping* concurrent unaligned IOs > where the kernel direct IO code causes the data corruption, not the > application. The application is not doing something stupid, and as > such needs to be fixed. ^^^^^^ the kernel bug needs to be fixed. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- 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
