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

[Michal Hocko <mhocko@suse.com>]

On Mon 13-01-25 18:19:17, 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. In theory we can
> introduce percpu reentrance counter and increment it every time
> spin_lock_irqsave(&zone->lock, flags) is used, but we cannot rely
> on it. Even if this cpu is not in page_alloc path the
> spin_lock_irqsave() is not safe, since BPF prog might be called
> from tracepoint where preemption is disabled. So trylock only.
> 
> Note, free_page and memcg are not taught about gfpflags_allow_spinning()
> condition. The support 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/
> 
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

LGTM, I am not entirely clear on kmsan_alloc_page part though.
As long as that part is correct you can add
Acked-by: Michal Hocko <mhocko@suse.com>

Other than that try_alloc_pages_noprof begs some user documentation.

/**
 * 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 but 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 succeess. Failures are not reported via warn_alloc().
 *
 * 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.
> +	 * 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;
> +
> +#ifdef CONFIG_UNACCEPTED_MEMORY
> +	/* Bailout, since try_to_accept_memory_one() needs to take a lock */
> +	if (has_unaccepted_memory())
> +		return NULL;
> +#endif
> +	/* Bailout, since _deferred_grow_zone() needs to take a lock */
> +	if (deferred_pages_enabled())
> +		return NULL;
> +
> +	if (nid == NUMA_NO_NODE)
> +		nid = numa_node_id();
> +
> +	prepare_alloc_pages(alloc_gfp, order, nid, NULL, &ac,
> +			    &alloc_gfp, &alloc_flags);
> +
> +	/*
> +	 * Best effort allocation from percpu free list.
> +	 * If it's empty attempt to spin_trylock zone->lock.
> +	 */
> +	page = get_page_from_freelist(alloc_gfp, order, alloc_flags, &ac);
> +
> +	/* Unlike regular alloc_pages() there is no __alloc_pages_slowpath(). */
> +
> +	trace_mm_page_alloc(page, order, alloc_gfp, ac.migratetype);
> +	kmsan_alloc_page(page, order, alloc_gfp);
> +	return page;
> +}
> -- 
> 2.43.5
-- 
Michal Hocko
SUSE Labs