linux-ext4 - Re: Journal under-reservation bug on first >2G file

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Message-ID: <20141002112645.GD19748@quack.suse.cz>
Date:	Thu, 2 Oct 2014 13:26:45 +0200
From:	Jan Kara <jack@...e.cz>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Theodore Ts'o <tytso@....edu>, Andreas Dilger <adilger@...ger.ca>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: Journal under-reservation bug on first >2G file

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@...e.cz>
SUSE Labs, CR
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