On 1/14/11 11:28 AM, Eric Sandeen wrote: > > > ext4 has a data corruption case when doing non-block-aligned > asynchronous direct IO into a sparse file, as demonstrated > by xfstest 240. > > The root cause is that while ext4 preallocates space in the > hole, mappings of that space still look "new" and > dio_zero_block() will zero out the unwritten portions. When > more than one AIO thread is going, they both find this "new" > block and race to zero out their portion; this is uncoordinated > and causes data corruption. > > Dave Chinner fixed this for xfs by simply serializing all > unaligned asynchronous direct IO. I've done the same here. > This is a very big hammer, and I'm not very pleased with > stuffing this into ext4_file_write(). But since ext4 is > DIO_LOCKING, we need to serialize it at this high level. > > I tried to move this into ext4_ext_direct_IO, but by then > we have the i_mutex already, and we will wait on the > work queue to do conversions - which must also take the > i_mutex. So that won't work. > > This was originally exposed by qemu-kvm installing to > a raw disk image with a normal sector-63 alignment. I've > tested a backport of this patch with qemu, and it does > avoid the corruption. It is also quite a lot slower > (14 min for package installs, vs. 8 min for well-aligned) > but I'll take slow correctness over fast corruption any day. > > Mingming suggested that perhaps we can track outstanding > conversions, and wait on that instead so that non-sparse > files won't be affected, but I've had trouble making that > work so far, and would like to get the corruption hole > plugged ASAP. Perhaps adding a prink_once() warning of > the perf degradation on this path would be useful? > I've sent a patch to do the above, as V3, twice now, but the list is eating it. Ted, you were cc'd so hopefully you got it? -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
