On Tue, Apr 29, 2008 at 10:10:59AM -0700, Zach Brown wrote: > > > The obvious fix for this is that block_write_begin() and > > friends should be calling ->setattr to do the truncation and hence > > follow normal convention for truncating blocks off an inode. > > However, even that appears to have thorns. e.g. in XFS we hold the > > iolock exclusively when we call block_write_begin(), but it is not > > held in all cases where ->setattr is currently called. Hence calling > > ->setattr from block_write_begin in this failure case will deadlock > > unless we also pass a "nolock" flag as well. XFS already > > supports this (e.g. see the XFS fallocate implementation) but no other > > filesystem does (some probably don't need to). > > This paragraph in particular reminds me of an outstanding bug with > O_DIRECT and ext*. It isn't truncating partial allocations when a dio > fails with ENOSPC. This was noticed by a user who saw that fsck found > bocks outside i_size in the file that saw ENOSPC if they tried to > unmount and check the volume after the failed write. That sounds very similar - ENOSPC seems to be one way of "easily" generating the error condition that exposes this condition, but I'm sure there are others as well... > So, whether we decide that failed writes should call setattr or > vmtruncate, we should also keep the generic O_DIRECT path in > consideration. Today it doesn't even try the supposed generic method of > calling vmtrunate(). Certainly, though the locking will certainly be entertaining in this path.... > (Though I'm sure XFS' dio code already handles freeing blocks :)) Not the dio code as such, but the close path does. Blocks beyond EOF get truncated off in ->release or ->clear_inode (unless they were specifically preallocated) and dio does not do delayed allocation so does not suffer from the "need ->setattr issue" to truncate them away on ENOSPC. i.e. after the error occurs and the app closes the fd, the blocks get truncated away. Basically the problem I described is leaving delayed allocation blocks beyond EOF without any page cache mappings to indicate they are there - allocated blocks beyond EOF are not a problem... Cheers, Dave. -- Dave Chinner Principal Engineer SGI Australian Software Group -- 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
