Re: [Ext2-devel] Re: [CHECKER] e2fsck writes out blocks out of order, causing root dir to be corrupted (ext3, linux 2.4.19, e2fsprogs 1.34)

From: Theodore Ts'o
Date: Tue May 11 2004 - 22:43:05 EST

On Tue, May 11, 2004 at 11:09:45PM -0400, Valdis.Kletnieks@xxxxxx wrote:
> I suspect this bug is merely a special case of "your filesystem can
> get scrogged if something's doing caching behind your back" - the
> same sort of issues that prompted recent "flush the IDE cache on
> shutdown" fixes, and the well-known issues with using journalling
> file systems on a file-backed loopback device.

No, not really. This is the case of "we moved code that was original
kernel-space to user space", and we failed to simulate certain
functions, such as sync_blockdev().

> Having said that, I admit being surprised that their demonstration
> test case is *that* simple - that's a truly small number of I/Os to
> get it into a repeatably corruptible state. I'm sure many of us
> have a mental image of these class of failures as being heisenbugs,
> dependent on the cache contents.

Well, the demonstration test case *wasn't* that simple. It required
the system crashing twice at very specific points. Once to create the
filesystem requiring a journal replay, and then a second crash at
exactly the right time in the middle of e2fsck's journal replaying
code. This failure would be fairly hard to replicate in real-life
conditions, since it would require to crashes in quick succession, at
very carefully chosen points, although if you had really flaky AC
mains, I suppose it might be considered a more likely failure case.

> Hardly - the class of errors is one that does (or should) concern
> the kernel community - and I don't consider identifying a "your
> filesystem *will* be toast if you get into this repeatable scenario"
> a troll. At the very least, we can consider what additional
> hardening we can do to either the kernel or userspace to make sure
> that we don't re-order the blocks - note the key phrase here:
> "Neither of these pay attention to the journaling constraints of
> EXT3 and JBD."

Well, actually it was the e2fsprogs user space code that wasn't paying
attention t the journalling constraints, mainly because we had been we
weren't faithfully implementing sync_blockdev()/fsync_no_super(). The
patch to fix this in e2fsprogs was fairly simple.

- Ted
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