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 <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Sebastian Andrzej Siewior <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

0 files changed, 0 insertions, 0 deletions