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).
This happens even for large allocations. In a prior iteration of the "When it's okay to treat OOM as fatal?" discussion[1], I showed that Linux malloc() and g_malloc() happily give me a terabyte of memory I don't have in 1024 chunks of 1 GiB each. I just reran the test, same results. See also [2]. [1] https://lists.gnu.org/archive/html/qemu-devel/2018-10/msg04229.html [2] http://turnoff.us/geek/bad-malloc/
