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

[Markus Armbruster]

"Dr. David Alan Gilbert" <address@hidden> writes:

> * Markus Armbruster (address@hidden) 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.