On Thu, 25 Nov 2010 14:53:56 +1100 Nick Piggin <npiggin@xxxxxxxxx> wrote: > On Wed, Nov 24, 2010 at 02:10:28PM +0100, Jan Kara wrote: > > On Wed 24-11-10 12:03:43, Nick Piggin wrote: > > > > For the _nr variant that btrfs uses, it's worse for the filesystems > > > > that don't have a 1:1 bdi<->sb mapping. It might not actually write any > > > > of the pages from the SB that is out of space. > > > > > > That's true, but it might not write anything anyway (and after we > > > check whether writeout is in progress, the writeout thread could go > > > to sleep and not do anything anyway). > > > > > > So it's a pretty hacky interface anyway. If you want to do anything > > > deterministic, you obviously need real coupling between producer and > > > consumer. This should only be a performance tweak (or a workaround > > > hack in worst case). > > Yes, the current interface is a band aid for the problem and better > > interface is welcome. But it's not trivial to do better... > > > > > > > It makes no further guarantees, and anyway > > > > > the sb has to compete for normal writeback within this bdi. > > > > > > > > > > > > > > I think Christoph is right because filesystems should not really > > > > > know about how bdi writeback queueing works. But I don't know if it's > > > > > worth doing anything more complex for this functionality? > > > > > > > > I think we should make a writeback_inodes_sb_unlocked() that doesn't > > > > warn when the semaphore isn't held. That should be enough given where > > > > btrfs and ext4 are calling it from. > > > > > > It doesn't solve the bugs -- calling and waiting for writeback is > > > still broken because completion requires i_mutex and it is called > > > from under i_mutex. > > Well, as I wrote in my previous email, only ext4 has the problem with > > i_mutex and I personally view it as a bug. But ultimately it's Ted's call > > to decide. > > Well, for now, the easiest and simplest fix is my patch, I think. The > objection is that we may not write out anything for the specified sb, > but the current implementation provides no such guarantees at all > anyway, so I don't think it's a big issue. Well yes. We take something which will fail occasionally and with your patch replace it with something which will fail a bit more often. Why don't we go all the way and do something which will fail *even more often*. Namely, just delete the damn function in the hope that the resulting failures will provoke the ext4 crew into doing something sane this time? Guys, this delalloc thing *sucks*. And here we are just sticking new bandaids on top of the old bandaids. And the btrfs approach isn't exactly a thing of glory, either. So... nope. I won't be applying Nick's patch. Please fix this thing properly - you have a whole month! -- 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
