On Thu, 2008-03-13 at 19:05 +0100, Jan Kara wrote: > Hi, > > As Mark Fasheh pointed out, we cannot take page_lock inside a transaction > commit because that could possibly deadlock with other thread holding the > page_lock and waiting for commit to finish in journal_start. This is > kind-of blocker for my new approach of handling of ordered mode in JBD. > > So first, I'd like to ask what other people think about reversing locking > order of page_lock and transaction start in ext3/4? I personally find it a > good thing anyway (I've stumbled on problems with the current locking order > several times, but so far I could always workaround them), logically it > simply "makes sence" as transaction handle is naturally more long-lived > than a lock on one page. > > For the case that we agree we want to reverse the order, I've looked into > how hard would it be. Ordinary write path is trivial. If we provide > page_mkwrite function (which should be quite simple), we don't have to be > afraid of instantiating holes in writepage so that makes things in > writepage simpler (although we'd pay the some performance for writing page > of zeros into the hole and later writing real data in writepage - currently > we do only the second write together with block allocation). With > page_mkwrite, we don't have to start transaction at all in writepage in > writeback and ordered modes. 3 years ago, I tried to support writepages() for ext3 and ran into this. At that time, page_mkwrite() was still under works and couldn't use it. I think with page_mkwrite() we can do this, except that its a slight behaviour change in the sense that the app could get allocation errors (ENOSPC) while writing to mmap(). But I think its a good thing. For ext4 delayed allocation, we might see small performance issue - but I think it would be a special case. Thanks, Badari -- 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
