* Markus Armbruster (arm...@redhat.com) wrote: > "Dr. David Alan Gilbert" <dgilb...@redhat.com> writes: > > > * Markus Armbruster (arm...@redhat.com) wrote: > >> We sometimes use g_new() & friends, which abort() on OOM, and sometimes > >> g_try_new() & friends, which can fail, and therefore require error > >> handling. > >> > >> HACKING points out the difference, but is mum on when to use what: > >> > >> 3. Low level memory management > >> > >> Use of the malloc/free/realloc/calloc/valloc/memalign/posix_memalign > >> APIs is not allowed in the QEMU codebase. Instead of these routines, > >> use the GLib memory allocation routines g_malloc/g_malloc0/g_new/ > >> g_new0/g_realloc/g_free or QEMU's > >> qemu_memalign/qemu_blockalign/qemu_vfree > >> APIs. > >> > >> Please note that g_malloc will exit on allocation failure, so there > >> is no need to test for failure (as you would have to with malloc). > >> Calling g_malloc with a zero size is valid and will return NULL. > >> > >> Prefer g_new(T, n) instead of g_malloc(sizeof(T) * n) for the following > >> reasons: > >> > >> a. It catches multiplication overflowing size_t; > >> b. It returns T * instead of void *, letting compiler catch more type > >> errors. > >> > >> Declarations like T *v = g_malloc(sizeof(*v)) are acceptable, though. > >> > >> Memory allocated by qemu_memalign or qemu_blockalign must be freed with > >> qemu_vfree, since breaking this will cause problems on Win32. > >> > >> Now, in my personal opinion, handling OOM gracefully is worth the > >> (commonly considerable) trouble when you're coding for an Apple II or > >> similar. Anything that pages commonly becomes unusable long before > >> allocations fail. > > > > That's not always my experience; I've seen cases where you suddenly > > allocate a load more memory and hit OOM fairly quickly on that hot > > process. Most of the time on the desktop you're right. > > > >> Anything that overcommits will send you a (commonly > >> lethal) signal instead. Anything that tries handling OOM gracefully, > >> and manages to dodge both these bullets somehow, will commonly get it > >> wrong and crash. > > > > If your qemu has maped it's main memory from hugetlbfs or similar pools > > then we're looking at the other memory allocations; and that's a bit of > > an interesting difference where those other allocations should be a lot > > smaller. > > > >> But others are entitled to their opinions as much as I am. I just want > >> to know what our rules are, preferably in the form of a patch to > >> HACKING. > > > > My rule is to try not to break a happily running VM by some new > > activity; I don't worry about it during startup. > > > > So for example, I don't like it when starting a migration, allocates > > some more memory and kills the VM - the user had a happy stable VM > > upto that point. Migration gets the blame at this point. > > I don't doubt reliable OOM handling would be nice. I do doubt it's > practical for an application like QEMU.
Well, our use of glib certainly makes it much much harder. I just try and make sure anywhere that I'm allocating a non-trivial amount of memory (especially anything guest or user controlled) uses the _try_ variants. That should keep a lot of the larger allocations. However, it scares me that we've got things that can return big chunks of JSON for example, and I don't think they're being careful about it. Dave -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
