Re: [PATCH bpf-next v4 1/6] mm, bpf: Introduce try_alloc_pages() for opportunistic page allocation

[Michal Hocko <mhocko@suse.com>]

On Tue 14-01-25 10:53:55, Peter Zijlstra wrote:
> On Mon, Jan 13, 2025 at 06:19:17PM -0800, Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@kernel.org>
> > 
> > Tracing BPF programs execute from tracepoints and kprobes where
> > running context is unknown, but they need to request additional
> > memory.
> 
> > The prior workarounds were using pre-allocated memory and
> > BPF specific freelists to satisfy such allocation requests.
> > Instead, introduce gfpflags_allow_spinning() condition that signals
> > to the allocator that running context is unknown.
> > Then rely on percpu free list of pages to allocate a page.
> > The rmqueue_pcplist() should be able to pop the page from.
> > If it fails (due to IRQ re-entrancy or list being empty) then
> > try_alloc_pages() attempts to spin_trylock zone->lock
> > and refill percpu freelist as normal.
> 
> > BPF program may execute with IRQs disabled and zone->lock is
> > sleeping in RT, so trylock is the only option. 
> 
> how is spin_trylock() from IRQ context not utterly broken in RT?

+	if (IS_ENABLED(CONFIG_PREEMPT_RT) && (in_nmi() || in_hardirq()))
+		return NULL;

Deals with that, right?

-- 
Michal Hocko
SUSE Labs