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

[Daniel P . Berrangé]

On Mon, Mar 15, 2021 at 05:54:17PM +0000, Alex Bennée wrote:
> 
> Daniel P. Berrangé <berrange@redhat.com> writes:
> 
> > 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).
> 
> OK how about this wording:
> 
>   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``). Generally using ``g_malloc`` on start-up is fine as the
>   result of a failure to allocate memory is going to be a fatal exit
>   anyway. There may be some start-up cases where failing is unreasonable
>   (for example speculatively loading a large debug symbol table).
> 
>   Care should be taken to avoid introducing places where the guest could
>   trigger an exit by causing a large allocation. For small allocations,
>   of the order of 4k, a failure to allocate is likely indicative of an
>   overloaded host and allowing ``g_malloc`` to ``exit`` is a reasonable
>   approach. However for larger allocations where we could realistically
>   fall-back to a smaller one if need be we should use functions like
>   ``g_try_new`` and check the result. For example this is valid approach
>   for a time/space trade-off like ``tlb_mmu_resize_locked`` in the
>   SoftMMU TLB code.

Fine with me

Regards,
Daniel
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