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

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Date:	Thu, 02 Oct 2014 00:49:09 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	"Theodore Ts'o" <tytso@....edu>
CC:	Andreas Dilger <adilger@...ger.ca>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: Journal under-reservation bug on first >2G file

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...

-Eric

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