mm, bpf: Introduce try_alloc_pages() for opportunistic page allocation

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.
try_alloc_pages() -> get_page_from_freelist() -> rmqueue() ->
rmqueue_pcplist() will spin_trylock to grab the page from percpu
free list. If it fails (due to re-entrancy or list being empty)
then rmqueue_bulk()/rmqueue_buddy() will attempt to
spin_trylock zone->lock and grab the page from there.
spin_trylock() is not safe in PREEMPT_RT when in NMI or in hard IRQ.
Bailout early in such case.

The support for gfpflags_allow_spinning() mode for free_page and memcg
comes in the next patches.

This is a first step towards supporting BPF requirements in SLUB
and getting rid of bpf_mem_alloc.
That goal was discussed at LSFMM: https://lwn.net/Articles/974138/

Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20250222024427.30294-3-alexei.starovoitov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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