Re: [Qemu-devel] When it's okay to treat OOM as fatal?

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Markus Armbruster (armbru@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 / dgilbert@redhat.com / Manchester, UK