* 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. Dave -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
