Daniel P. Berrangé <berra...@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.ben...@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
