"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.
