Re: Error handling: How to "lose" a transaction

[Jeff Mahoney <jeffm@suse.de>]

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On 12/21/2011 10:21 PM, Liu Bo wrote:
> On 12/22/2011 10:59 AM, Jeff Mahoney wrote: Sorry I haven't
> responded to this yet. I started digging right in and I've started
> to have some good results. It turns out there's already a 
> btrfs_cleanup_transaction call that will tear down outstanding 
> transactions. It's not perfect and I've fixed a few bugs in there,
> but it saved me a bunch of effort. I just wished I noticed it a day
> before since I had it half implemented myself. :)
> 
> 
>> Hi Jeff,
> 
>> Yes, it should be, and I wrote this cleanup_transaction where I
>> should notice you earlier... Anyway, thanks for your effort.
> 
>> The error handling part has lots of corner cases, so I just pick
>> up a brute way to tear down the current transaction in order to
>> make the FS RO.

Oh, and it's worked great. The brute force method is a good start and
will address the most severe problems (and most cases) well. I've
decided to ignore most cases of -ENOMEM for now. The biggest bug I ran
into so far was calling mutex_lock while holding a spinlock. It was a
quick fix.

The method I've generally used is to mark the transaction aborted and
pass the error up as quickly as possible, cleaning up the local
allocations and locks as I go. The transaction gets completed
normally, returns an error, isn't committed, and then is destroyed
(with others, potentially) when called from in
btrfs_commit_transaction. Btrfs makes this super easy since we can
just skip all the CoW writes.

Thanks!

- -Jeff


>> thanks, liubo
> 
> This afternoon I started running xfstests on a dm-linear mapped 
> partition. Halfway through a sufficiently long test, I swap out
> the linear mapping to an error mapping. It still crashes, but
> somewhat less spectacularly. There are still a ton of BUG_ON's I
> need to eliminate as well as work out the usual I/O error-recovery
> issue of uninterruptible, unrecoverable writeback contexts and
> still-locked pages holding up exit. I'm pretty pleased with the
> results so far and am pretty optimistic.
> 
> -Jeff
> 
> 
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> 

- -- 
Jeff Mahoney
SUSE Labs
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