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

[Alex Bennée]

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.


>
> Regards,
> Daniel


-- 
Alex Bennée