On 2025-01-14 18:17:40 [-0800], Alexei Starovoitov wrote: > From: Alexei Starovoitov <ast@xxxxxxxxxx> > > 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 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@xxxxxxxx> Acked-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> could you… > Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx> … > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 1cb4b8c8886d..74c2a7af1a77 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -7023,3 +7032,86 @@ static bool __free_unaccepted(struct page *page) > } > > #endif /* CONFIG_UNACCEPTED_MEMORY */ > + > +/** > + * try_alloc_pages_noprof - opportunistic reentrant allocation from any context > + * @nid - node to allocate from > + * @order - allocation order size > + * > + * Allocates pages of a given order from the given node. This is safe to > + * call from any context (from atomic, NMI, and also reentrant > + * allocator -> tracepoint -> try_alloc_pages_noprof). > + * Allocation is best effort and to be expected to fail easily so nobody should > + * rely on the success. Failures are not reported via warn_alloc(). Could you maybe add a pointer like "See AlwaysFailRestrictions below." or something similar to make the user aware of the comment below where certain always-fail restrictions are mentioned. Such as PREEMPT_RT + NMI or deferred_pages_enabled(). It might not be easy to be aware of this. I'm curious how this turns out in the long run :) > + * > + * Return: allocated page or NULL on failure. > + */ > +struct page *try_alloc_pages_noprof(int nid, unsigned int order) > +{ > + /* > + * Do not specify __GFP_DIRECT_RECLAIM, since direct claim is not allowed. > + * Do not specify __GFP_KSWAPD_RECLAIM either, since wake up of kswapd > + * is not safe in arbitrary context. > + * > + * These two are the conditions for gfpflags_allow_spinning() being true. > + * > + * Specify __GFP_NOWARN since failing try_alloc_pages() is not a reason > + * to warn. Also warn would trigger printk() which is unsafe from > + * various contexts. We cannot use printk_deferred_enter() to mitigate, > + * since the running context is unknown. > + * > + * Specify __GFP_ZERO to make sure that call to kmsan_alloc_page() below > + * is safe in any context. Also zeroing the page is mandatory for > + * BPF use cases. > + * > + * Though __GFP_NOMEMALLOC is not checked in the code path below, > + * specify it here to highlight that try_alloc_pages() > + * doesn't want to deplete reserves. > + */ > + gfp_t alloc_gfp = __GFP_NOWARN | __GFP_ZERO | __GFP_NOMEMALLOC; > + unsigned int alloc_flags = ALLOC_TRYLOCK; > + struct alloc_context ac = { }; > + struct page *page; > + > + /* > + * In RT spin_trylock() may call raw_spin_lock() which is unsafe in NMI. PREEMPT_RT please. s/may/will > + * If spin_trylock() is called from hard IRQ the current task may be > + * waiting for one rt_spin_lock, but rt_spin_trylock() will mark the > + * task as the owner of another rt_spin_lock which will confuse PI > + * logic, so return immediately if called form hard IRQ or NMI. > + * > + * Note, irqs_disabled() case is ok. This function can be called > + * from raw_spin_lock_irqsave region. > + */ > + if (IS_ENABLED(CONFIG_PREEMPT_RT) && (in_nmi() || in_hardirq())) > + return NULL; > + if (!pcp_allowed_order(order)) > + return NULL; … Sebastian
