* Markus Armbruster (arm...@redhat.com) wrote: > Paolo Bonzini <pbonz...@redhat.com> writes: > > > On 09/12/2015 10:30, Markus Armbruster wrote: > >> 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. > > > > I suspect a lot of memory_region_init_ram()s could be considered > > potentially large (at least in the 16-64 megabytes range). Propagation > > of memory_region_init_ram() failures is easy enough, thanks to Error**, > > that we should just do it. > > Propagating an out-of-memory error right in realize() is easy. What's > not so easy is making realize() fail cleanly (all side effects undone; > we get that wrong in many places), and finding and propagating > out-of-memory errors hiding deeper in the call tree. > > However, genuinely "large" allocations should be relatively few, and > handling them gracefully in hot-pluggable devices is probably feasible. > > I doubt ensuring *all* allocations on behalf of a hot-pluggable device > are handled gracefully is a good use of our reseources, or even > feasible. > > Likewise, graceful error handling for devices that cannot be hot-plugged > feels like a waste of resources we can ill afford. I think we should > simply document their non-gracefulness by either setting hotpluggable = > false or cannot_instantiate_with_device_add_yet = true with a suitable > comment. > > > Even if we don't, we should use &error_abort, not &error_fatal > > (programmer error---due to laziness---rather than user error). > > &error_fatal should really be restricted to code that is running very > > close to main(). > > "Very close to main" is a question of dynamic context. > > Consider a device that can only be created during machine initialization > (cannot_instantiate_with_device_add_yet = true or hotpluggable = false). > &error_fatal is perfectly adequate there. &error_abort would be > misleading, because when it fails, it's almost certainly because the > user tried to create too big a machine. > > Now consider a hot-pluggable device. Its recognized "large" allocations > all fail gracefully. What about its other allocations? Two kinds: the > ones visible in the device model code, and the ones hiding elsewhere, > which include "a few" of the 2300+ uses of GLib memory allocation. The > latter exit(). Why should the former abort()? > > Now use that hot-pluggable device during machine initialization. > abort() is again misleading. > > Let's avoid a fruitless debate on when to exit() and when to abort() on > out-of-memory, and just stick to exit(). We don't need a core dump to > tell a developer to fix his lazy error handling.
The tricky bit is when a user says 'it crashed with out of memory' - and we just did an exit, how do we find out which bit we should improve the error handling on? I guess the use of abort() could tell us that - however it's a really big assumption that in an OOM case we'd be able to dump the information. Dave -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
