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Re: [RFC PATCH] docs/devel: expand style section of memory management
From: |
Daniel P . Berrangé |
Subject: |
Re: [RFC PATCH] docs/devel: expand style section of memory management |
Date: |
Mon, 15 Mar 2021 18:06:25 +0000 |
User-agent: |
Mutt/2.0.5 (2021-01-21) |
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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