linux-ext4 - Re: Fast symlinks stored slow

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Date:   Wed, 12 Jul 2017 19:17:37 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     "Richard W.M. Jones" <rjones@...hat.com>
Cc:     linux-ext4@...r.kernel.org, Tahsin Erdogan <tahsin@...gle.com>
Subject: Re: Fast symlinks stored slow

On Wed, Jul 12, 2017 at 06:07:11PM +0100, Richard W.M. Jones wrote:
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=1470157
> 
> To cut a long story short, we were using libext2fs to create
> filesystems where short symlinks (< 60 bytes) were stored the same way
> as long symlinks, ie. stored as an ordinary file instead of being
> stored in the inode.
> 
> I think the reason we were creating filesystems wrongly in the first
> place is because our code has been around since about 2008, and the
> nice ext2fs_symlink function that deals properly with fast/slow
> symlinks wasn't added until 2013.

Thanks for the report.  I had been hesitant about making this change
(and had been pushing back from those who were advocating for this
change) precisely because I was afraid that this might be a situation.

What convinced me to accept the change is that (a) I had scanned all
of the old kernels and old versions of e2fsprogs and convinced myself
that aside from someone manually creating symlinks using low-level
libext2fs, symlinks < 60 bytes would never be stored in external
blocks, and (b) using the i_blocks logic to determine whether or not
we had a slow link was getting really painful.

> It's not too much trouble for us to recreate the incorrect
> filesystems.  Mostly we're creating one-off throwaway filesystems for
> appliances anyway and they don't live for long.
> 
> But I suppose this might be a warning that other incorrect filesystems
> exist which will break with Linux >= 4.13.

So I see this is going to break libvert and libguestfs.  So people who
are running existing distribution userspaces and then upgrade to 4.13
will break.

Hmm...  I suppose we could add back support to let the kernel to use
the i_blocks logic if the ea_inode feature is not enabled.  E2fsck
would still complain so we can try to gradually force userspace to do
things "correctly", but at least we would be backwards compatible.

Comments?

						- Ted