On Thu 02-10-14 00:49:09, Eric Sandeen wrote: > On 10/1/14 5:43 PM, Theodore Ts'o wrote: > > On Wed, Oct 01, 2014 at 03:37:17PM -0500, Eric Sandeen wrote: > >> > >> Ok. I guess this is only an issue for ext4 - well, at least this specific > >> issue. Delalloc makes it much different than ext2 & ext3, which reserve quite a > >> lot more. Whether there's a corner case over there which breaks, I dunno... > >> > >> So it seems like the simplest test is simply: Are we RW mounted with delalloc? > >> And if so, update the feature. Seems simpler than mucking with "which features > >> are unique to ext4" > > > > I'd do "are we RW mounted with the extents feature". That way we > > don't need to worry about someone accidentally mounting a partition > > meant for Hurd using ext4, which would imply delalloc, and then > > causing Hurd to no longer be able to deal with the file system. That > > *shouldn't* happen, but if someone accidentally mounts the file system > > with -t ext4, but it seems safer to gate it on the existence of the > > extents feature. > > Problem is, we can hit the same problem with an ext3 filesystem (no > extents) mounted with -t ext4 (enabling delalloc). > > Ugh. Can't we just bump the da write reservation to 2 and be done with it? ;) > (AFAICT the non-delalloc reservations can be wildly overestimated). > > Or maybe ext4_journal_extend() when we try to update the superblock? > It could fail, but it wouldn't be catastrophic if it did, fsck would find > that the feature is missing... A couple of notes: 1) Using 2 would be fine. Journal code is clever enough and it returns unused handle credits to the transaction so using 2 instead of 1 limits only the number of handles in ext4_da_write_begin() running in parallel. So I'd frankly just bump the number to 2 (with a comment!) and be done with it. 2) If we want to optimize a bit, we can check whether the write is going to extend beyond 2G and first set the feature in a separate transaction. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- 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
