On Thu, 26 Jul 2007 04:36:39 +0200 Nick Piggin <[EMAIL PROTECTED]> wrote:
[ are state trees a good idea? ] > > One thing it gains us is finding the start of the cluster. Even if > > called by kswapd, the state tree allows writepage to find the start > > of the cluster and send down a big bio (provided I implement > > trylock to avoid various deadlocks). > > That's very true, we could potentially also do that with the block > extent tree that I want to try with fsblock. If fsblock records and extent of 200MB, and writepage is called on a page in the middle of the extent, how do you walk the radix backwards to find the first dirty & up to date page in the range? > > I'm looking at "cleaning up" some of these aops APIs so hopefully > most of the deadlock problems go away. Should be useful to both our > efforts. Will post patches hopefully when I get time to finish the > draft this weekend. Great > > > > > > O_DIRECT becomes a special case of readpages and > > > > writepages....the memory used for IO just comes from userland > > > > instead of the page cache. > > > > > > Could be, although you'll probably also need to teach the mm about > > > the state tree and/or still manipulate the pagecache tree to > > > prevent concurrency? > > > > Well, it isn't coded yet, but I should be able to do it from the FS > > specific ops. > > Probably, if you invalidate all the pagecache in the range beforehand > you should be able to do it (and I guess you want to do the invalidate > anyway). Although, below deadlock issues might still bite somehwere... Well, O_DIRECT is french for deadlocks. But I shouldn't have to worry so much about evicting the pages themselves since I can tag the range. > > > > > But isn't the main aim of O_DIRECT to do as little locking and > > > synchronisation with the pagecache as possible? I thought this is > > > why your race fixing patches got put on the back burner (although > > > they did look fairly nice from a correctness POV). > > > > I put the placeholder patches on hold because handling a corner case > > where userland did O_DIRECT from a mmap'd region of the same file > > (Linus pointed it out to me). Basically my patches had to work in > > 64k chunks to avoid a deadlock in get_user_pages. With the state > > tree, I can allow the page to be faulted in but still properly deal > > with it. > > Oh right, I didn't think of that one. Would you still have similar > issues with the external state tree? I mean, the filesystem doesn't > really know why the fault is taken. O_DIRECT read from a file into > mmapped memory of the same block in the file is almost hopeless I > think. Racing is fine as long as we don't deadlock or expose garbage from disk. > > > > The ability to put in additional tracking info like the process > > > > that first dirtied a range is also significant. So, I think it > > > > is worth trying. > > > > > > Definitely, and I'm glad you are. You haven't converted me yet, > > > but I look forward to finding the best ideas from our two > > > approaches when the patches are further along (ext2 port of > > > fsblock coming along, so we'll be able to have races soon :P). > > > > I'm sure we can find some river in Cambridge, winner gets to throw > > Axboe in. > > Very noble of you to donate your colleage to such a worthy cause. Jens is always interested in helping solve such debates. It's a fantastic service he provides to the community. -chris - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
