Re: [RFC PATCH] docs/devel: expand style section of memory management

[Daniel P . Berrangé]

On Mon, Mar 15, 2021 at 06:04:10PM +0100, Thomas Huth wrote:
> On 15/03/2021 17.57, Peter Maydell wrote:
> > On Mon, 15 Mar 2021 at 16:53, Alex Bennée <alex.bennee@linaro.org> wrote:
> > > -Prefer g_new(T, n) instead of g_malloc(sizeof(T) ``*`` n) for the 
> > > following
> > > +Care should be taken to avoid introducing places where the guest could
> > > +trigger an exit. For example using ``g_malloc`` on start-up is fine
> > > +if the result of a failure is going to be a fatal exit anyway. There
> > > +may be some start-up cases where failing is unreasonable (for example
> > > +speculatively loading debug symbols).
> > > +
> > > +However if we are doing an allocation because of something the guest
> > > +has done we should never trigger an exit. The code may deal with this
> > > +by trying to allocate less memory and continue or re-designed to allocate
> > > +buffers on start-up.
> > 
> > I think this is overly strong. We want to avoid malloc-or-die for
> > cases where the guest gets to decide how big the allocation is;
> > but if we're doing a single small fixed-size allocation that happens
> > to be triggered by a guest action we should be OK to g_malloc() that
> > I think.
> 
> I agree with Peter. If the host is so much out-of-memory that we even can't
> allocate some few bytes anymore (let's say less than 4k), the system is
> pretty much dead anyway and it might be better to terminate the program
> immediately instead of continuing with the out-of-memory situation.

On a Linux host you're almost certainly not going to see g_malloc
fail for small allocations at least. Instead at some point the host
will be under enough memory pressure that the OOM killer activates
and reaps arbitrary processes based on some criteria it has, freeing
up memory for malloc to succeed (unless OOM killer picked you as the
victim).

Regards,
Daniel
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