On Tue, Apr 16 2013 at 1:24pm -0400, Tejun Heo <tj@xxxxxxxxxx> wrote: > Hey, > > On Mon, Apr 15, 2013 at 09:02:06AM -0400, Mikulas Patocka wrote: > > The patch is not bug-prone, because we already must make sure that the > > cloned bio has shorter lifetime than the master bio - so the patch doesn't > > introduce any new possibilities to make bugs. > > The whole world isn't composed of only your code. As I said > repeatedly, you're introducing an API which is misleading and can > easily cause subtle bugs which are very difficult to reproduce. > > Imagine it being used to tag a metatdata or checksum update bio being > sent down while processing another bio and used to "clone" the context > of the original bio. It'll work most of the time even if the original > bio gets completed first but it'll break when it gets really unlucky - > e.g. racing with other operations which can put the base css ref, and > it'll be hellish to reproduce and everyone would have to pay for your > silly hack. > > > > Do the two really look the same to you? The page refs are much more > > > expensive, mostly contained in and the main focus of dm. ioc/css refs > > > aren't that expensive to begin with, css refcnting is widely scattered > > > > ioc is per-task, so it is likely to be cached (but there are processors > > that have slow atomic operations even on cached data - on Pentium 4 it > > takes about 100 cycles). But css is shared between tasks and produces the > > cache ping-pong effect. > > For $DIETY's sake, how many times do I have to tell you to use per-cpu > reference count? Why do I have to repeat the same story over and over > again? What part of "make the reference count per-cpu" don't you get? > It's not a complicated message. > > At this point, I can't even understand why or what the hell you're > arguing. There's a clearly better way to do it and you're just > repeating yourself like a broken record that your hack in itself isn't > broken. > > So, if you wanna continue that way for whatever reason, you have my > firm nack and I'm outta this thread. > > Bye bye. Hey Tejun, I see you nack and raise you with: please reconsider in the near term. Your point about not wanting to introduce a generic block interface that isn't "safe" for all users noted. But as Mikulas has repeatedly said DM does _not_ ever need to do the refcounting. So it seems a bit absurd to introduce the requirement that DM should stand up an interface that uses percpu. That is a fair amount of churn that DM will never have a need to take advantage of. So why not introduce __bio_copy_association(bio1, bio2) and add a BUG_ON in it if bio2 isn't a clone of bio1? When there is a need for async IO to have more scalable refcounting that would be the time to introduce bio_copy_association that uses per-cpu refcounting (and yes we could then even nuke __bio_copy_association). It just seems to me a bit burdensome to ask Mikulas to add this infrastructure when DM really doesn't need it at all. But again I do understand your desire for others to be stearing the kernel where it needs to be to benefit future use-cases. But I think in general it best to introduce complexity when there is an actual need. Your insights are amazingly helpful and I think it is unfortunate that this refcounting issue overshadowed the positive advancements of dm-crypt scaling. I'm just looking to see if we can carry on with a temporary intermediate step with __bio_copy_association. Thanks, Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
