On 08/11/2011 11:28 AM, Jan Kara wrote: > Hello, > > On Thu 11-08-11 09:32:22, Josef Bacik wrote: >> I have this weird bug that has been plaguing me for a while where >> t_outstanding_credits will end up less than t_nr_buffers. I have done >> all sorts of things to try and catch when it happens but nothing seems >> to catch it. At some point I had thought that we were screwing up in >> journal_unmap_buffer. If a buffer is not on a transaction but is still >> a part of a checkpoint we will do a journal_file_buffer() onto the >> current running transaction's forget list. The thing is we can still >> have b_modified set since we only clear it on >> do_get_write_access/journal_get_create_access if it isn't a part of the >> transaction yet. So if we do the journal_file_buffer() before anybody >> calls do_get_write_access/journal_get_create_access we will short >> circuit these checks and b_modified will never be cleared and so when we >> do journal_dirty_metadata we won't account for the new buffer and it >> will end up inc'ing t_nr_buffers but not t_outstanding_credits. > Good spotting! > >> I had thought this was the problem before and put in a jh->b_modified = >> 0 in __dispose_buffer, but apparently the problem still happened. But >> that support person/customer were not entirely reliable so I'm back to >> thinking this is what happened and they just didn't run with my patch. > Umm, I think there's one more way how buffer b_modified == 1 can get > to other transaction's forget list. In journal_unmap_buffer(), transaction > == journal->j_committing_transaction case we do set_buffer_freed() and > set b_next_transaction to the running transaction. So when the currently > committing transaction finishes, it refiles the buffer to BJ_Forget list > of the running transaction. b_modified handling seems to be really fragile > in this regard. I guess the rule is that whenever we are going to change > b_transaction or b_next_transaction, we should clear b_modified. > Well this is happening on RHEL5, where we have set_buffer_freed(); if (jh->b_next_transaction) jh->b_next_transaction = NULL; so the only way this happens if it goes through __dispose_buffer. And the more I look at this I can't see how it would happen exactly. I can definitely get a modified buffer to show up on the forget list, but I can't see how I would then re-modify the thing to get it to show up on BJ_Metadata. On data=journal mode I can definitely see how to do it, but not with data=ordered mode. The only way to go through journal_unmap_buffer is to truncate the inode, and for a symlink the only way to make that happen is to delete it. So I don't see how I could then make it get dirtied again? Thanks, Josef -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
