Re: 2.6.18 ext3 panic.

[Eric Sandeen <esandeen@redhat.com>]

Badari Pulavarty wrote:

> Here is what I think is happening..
>
> journal_unmap_buffer() - cleaned the buffer, since its outside EOF, but
> its a part of the same page. So it remained on the page->buffers
> list. (at this time its not part of any transaction).
>
> Then, ordererd_commit_write() called journal_dirty_data() and we added
> all these buffers to BJ_SyncData list. (at this time buffer is clean -
> not dirty).
>
> Now msync() called __set_page_dirty_buffers() and dirtied *all* the
> buffers attached to this page.
>
> journal_submit_data_buffers() got around to this buffer and tried to
> submit the buffer...

This seems about right, but one thing bothers me in the traces; it seems like 
there is some locking that is missing.  In
http://people.redhat.com/esandeen/traces/eric_ext3_oops1.txt
for example, it looks like journal_dirty_data gets started, but then the 
buffer_head is acted on by journal_unmap_buffer, which decides this buffer is 
part of the running transaction, past EOF, and clears mapped, dirty, etc.  Then 
journal_dirty_data picks up again, decides that the buffer is not on the right 
list (now BJ_None) and puts it back on BJ_SyncData.  Then it gets picked up by 
journal_submit_data_buffers and submitted, and oops.

Talking with Stephen, it seemed like the page lock should synchronize these 
threads, but I've found that we can get to journal_dirty_data acting on the 
buffer heads w/o having the page locked...

I'm still digging, and, er, grasping at straws here... Am I off base?

-Eric


> Andrew is right - only option for us to check the filesize in the
> write out path and skip the buffers beyond EOF.
>
> Thanks,
> Badari

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