On Wed, Oct 10, 2012 at 02:43:15PM -0700, Zach Brown wrote: > > True. But that could be solved with a separate interface that either > > doesn't use a context to submit a call synchronously, or uses an > > implicit per thread context. > > Sure, but why bother if we can make the one submission interface fast > enough to satisfy quick callers? Less is more, and all that. Very true, if it's possible. I'm just still skeptical. > > I don't have a _strong_ opinion there, but my intuition is that we > > shouldn't be creating new types of handles without a good reason. I > > don't think the annoyances are for the most part particular to file > > descriptors, I think the tend to be applicable to handles in general and > > at least with file descriptors they're known and solved. > > I strongly disagree. That descriptors are an expensive limited > resources is a perfectly good reason to not make them required to access > the ring. What's so special about aio vs. epoll, and now signalfd/eventfd/timerfd etc.? > > That would be awesome, though for it to be worthwhile there couldn't be > > any kernel notion of a context at all and I'm not sure if that's > > practical. But the idea hadn't occured to me before and I'm sure you've > > thought about it more than I have... hrm. > > > > Oh hey, that's what acall does :P > > :) > > > For completions though you really want the ringbuffer pinned... what do > > you do about that? > > I don't think the kernel has to mandate that, no. The code has to deal > with completions faulting, but they probably won't. In acall it > happened that completions always came from threads that could block so > its coping mechanism was to just use put_user() :). Yeah, but that means the completion has to be delivered from process context. That's not what aio does today, and it'd be a real performance regression. I don't know of a way around that myself. > If userspace wants them rings locked, they can mlock() the memory. > > Think about it from another angle: the current mechanism of creating an > aio ring is a way to allocate pinned memory outside of the usual mlock > accounting. This could be abused, so aio grew an additional tunable to > limit the number of total entries in rings in the system. > > By putting the ring in normal user memory we avoid that problem > entirely. No different from any other place the kernel allocates memory on behalf of userspace... it needs a general solution, not a bunch of special case solutions (though since the general solution is memcg you might argue the cure is worse than the disease... :P) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
