* Markus Armbruster (arm...@redhat.com) wrote: > "Dr. David Alan Gilbert" <dgilb...@redhat.com> writes: > > > * Markus Armbruster (arm...@redhat.com) wrote: > >> In general, code running withing a realize() method should not exit() on > >> error. Instad, errors should be propagated through the realize() > >> method. Additionally, the realize() method should fail cleanly, > >> i.e. carefully undo its side effects such as wiring of interrupts, > >> mapping of memory, and so forth. Tedious work, but necessary to make > >> hot plug safe. > [...] > >> Next, let's consider the special case "out of memory". > >> > >> Our general approach is to treat it as immediately fatal. This makes > >> sense, because when a smallish allocation fails, the process is almost > >> certainly doomed anyway. Moreover, memory allocation is frequent, and > >> attempting to recover from failed memory allocation adds loads of > >> hard-to-test error paths. These are *dangerous* unless carefully tested > >> (and we don't). > >> > >> Certain important allocations we handle more gracefully. For instance, > >> we don't want to die when the user tries to hot-plug more memory than we > >> can allocate, or tries to open a QCOW2 image with a huge L1 table. > >> > >> Guest memory allocation used to have the "immediately fatal" policy > >> baked in at a fairly low level, but it's since been lifted into callers; > >> see commit c261d77..fc7a580 and fixups 4f96676..0bdaa3a. During review > >> of the latter, Peter Crosthwaite called out the &error_fatal in the > >> realize methods and their supporting code. I agreed with him back then > >> that the errors should really be propagated. But now I've changed my > >> mind: I think we should treat these memory allocation failures like > >> we've always treated them, namely report and exit(1). Except for > >> "large" allocations, where we have a higher probability of failure, and > >> a more realistic chance to recover safely. > >> > >> Can we agree that passing &error_fatal to memory_region_init_ram() & > >> friends is basically okay even in realize() methods and their supporting > >> code? > > > > I'd say it depends if they can be hotplugged; I think anything that we > > really > > want to hotplug onto real running machines (as opposed to where we're just > > hotplugging during setup) we should propagate errors properly. > > > > And tbh I don't buy the small allocation argument; I think we should handle > > them > > all; in my utopian world a guest wouldn't die unless there was no way out. > > I guess in Utopia nobody ever makes stupid coding mistakes, the error > paths are all covered by a comprehensive test suite, and they make the > code prettier, too. Oh, and kids always eat their vegetables without > complaint.
Yes, it's lovely. > However, we don't actually live in Utopia. In our world, error paths > clutter the code, are full of bugs, and the histogram of their execution > counts in testing (automated or not) has a frighteningly tall bar at > zero. > > We're not going to make this problem less severe by making it bigger. > In fact, we consciously decided to hack off a big chunk with an axe: > > commit 8a1d02aba9f986ca03d854184cd432ee98bcd179 > Author: aliguori <aliguori@c046a42c-6fe2-441c-8c8c-71466251a162> > Date: Thu Feb 5 22:05:49 2009 +0000 > > Terminate emulation on memory allocation failure (Avi Kivity) > > Memory allocation failures are a very rare condition on virtual-memory > hosts. They are also very difficult to handle correctly (especially in a > hardware emulation context). Because of this, it is better to gracefully > terminate emulation rather than executing untested or even unwritten > recovery > code paths. > > This patch changes the qemu memory allocation routines to terminate > emulation > if an allocation failure is encountered. > > Signed-off-by: Avi Kivity <a...@redhat.com> > Signed-off-by: Anthony Liguori <aligu...@us.ibm.com> > > > git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6526 > c046a42c-6fe2-441c-8c8c-71466251a162 > > Let me elaborate a bit on Avi's arguments: > > * Memory allocations are very, very common. I count at least 2500, > Memory allocation failure is easily the most common *potential* error, > both statically and dynamically. > > * Error recovery is always tedious and often hard. Especially when the > error happens in the middle of a complex operation that needs to be > fully rolled back to recover. A sensible approach is to acquire > resources early, when recovery is still relatively easy, but that's > often impractical for memory. This together with the ubiquitousness > of memory allocation makes memory allocation failure even harder to > handle than other kinds of errors. > > * Not all allocations are equal. When an attempt to allocate a gigabyte > fails gracefully, there's a good chance that ordinary code can go on > allocating and freeing kilobytes as usual. But when an attempt to > allocate kilobytes fails, it's very likely that handling this failure > gracefully will only lead to another one, and another one, until some > buggy error handling puts the doomed process out of its misery. > > Out of the ~2400 memory allocations that go through GLib, 59 can fail. > The others all terminate the process. > > * How often do we see failures from these other 2300+? Bug reports from > users? As far as I can see, they're vanishingly rare. > > * Reviewing and testing the error handling chains rooted at 59 > allocations is hard enough, and I don't think we're doing particularly > well on the testing. What chance would we have with 2300+ more? > > 2300+ instances of a vanishingly rare error with generally complex > error handling and basically no test coverage: does that sound more > useful than 2300+ instances of a vanishingly rare error that kills the > process? If yes, how much of our limited resources is the difference > worth? > > * You might argue that we don't have to handle all 2300+ instances, only > the ones reachable from hotplug. I think that's a pipe dream. Once > you permit "terminate on memory allocation failure" in general purpose > code, it hides behind innocent-looking function calls. Even if we > could find them all, we'd still have to add memory allocation failure > handling to lots of general purpose code just to make it usable for > hot plug. And then we'd get to keep finding them all forever. I didn't say it was easy :-) > I don't think handling all memory allocation failures gracefully > everywhere or even just within hotplug is practical this side of Utopia. > I believe all we could achieve trying is an illusion of graceful > handling that is sufficiently convincing to let us pat on our backs, > call the job done, and go back to working on stuff that matters to > actual users. Handling them all probably isn't; handling some well defined cases is probably possible. Avi's argument is 6 years old, I suggest a few things have changed in that time: a) We now use the Error** mechanism in a lot of places - so a lot of code already is supposed to deal with a function call failing; if a function already has an error return and the caller deals with it, then making the function deal with an allocation error and the caller handle it is a lot easier. b) The use of hotplug is now common - people really hate it when their nice, happy working VM dies when they try and do something to it, like hotplug or migrate. c) I suspect (but don't know) that people are pushing the number of VMs on a host harder than they used to, but there again memory got cheap. I'm not that eager to protect every allocation; but in some common cases, where we already have Error** paths and it's relatively simple, then I think we should. (OK, to be honest I think we should protect every allocation - but I do have sympathy with the complexity/testing arguments). Dave > My current working assumption is that passing &error_fatal to > memory_region_init_ram() & friends is okay even in realize() methods and > their supporting code, except when the allocation can be large. Even > then, &error_fatal is better than buggy recovery code (which I can see > all over the place, but that's a separate topic). -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
