Re: [RFC PATCH v1 13/57] bpf: Remove PAGE_SIZE compile-time constant assumption

[Ryan Roberts <ryan.roberts@arm.com>]

+ Andrii Nakryiko

This was a rather tricky series to get the recipients correct for and my script
did not realize that "supporter" was a pseudonym for "maintainer" so you were
missed off the original post. Appologies!

More context in cover letter:
https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/


On 14/10/2024 11:58, Ryan Roberts wrote:
> To prepare for supporting boot-time page size selection, refactor code
> to remove assumptions about PAGE_SIZE being compile-time constant. Code
> intended to be equivalent when compile-time page size is active.
> 
> Refactor "struct bpf_ringbuf" so that consumer_pos, producer_pos,
> pending_pos and data are no longer embedded at (static) page offsets
> within the struct. This can't work for boot-time page size because the
> page size isn't known at compile-time. Instead, only define the meta
> data in the struct, along with pointers to those values. At "struct
> bpf_ringbuf" allocation time, the extra pages are allocated at the end
> and the pointers are initialized to point to the correct locations.
> 
> Additionally, only expose the __PAGE_SIZE enum to BTF for compile-time
> page size builds. We don't know the page size at compile-time for
> boot-time builds. NOTE: This may need some extra thought; perhaps
> __PAGE_SIZE should be exposed as 0 in this case? And/or perhaps
> __PAGE_SIZE_MIN/__PAGE_SIZE_MAX should be exposed? And there would need
> to be a runtime mechanism for querying the page size (e.g.
> getpagesize()).
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
> 
> ***NOTE***
> Any confused maintainers may want to read the cover note here for context:
> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/
> 
>  kernel/bpf/core.c    |  9 ++++++--
>  kernel/bpf/ringbuf.c | 54 ++++++++++++++++++++++++--------------------
>  2 files changed, 37 insertions(+), 26 deletions(-)
> 
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 7ee62e38faf0e..485875aa78e63 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -89,10 +89,15 @@ void *bpf_internal_load_pointer_neg_helper(const struct sk_buff *skb, int k, uns
>  	return NULL;
>  }
>  
> -/* tell bpf programs that include vmlinux.h kernel's PAGE_SIZE */
> +/*
> + * tell bpf programs that include vmlinux.h kernel's PAGE_SIZE. We can only do
> + * this for compile-time PAGE_SIZE builds.
> + */
> +#if PAGE_SIZE_MIN == PAGE_SIZE_MAX
>  enum page_size_enum {
>  	__PAGE_SIZE = PAGE_SIZE
>  };
> +#endif
>  
>  struct bpf_prog *bpf_prog_alloc_no_stats(unsigned int size, gfp_t gfp_extra_flags)
>  {
> @@ -100,7 +105,7 @@ struct bpf_prog *bpf_prog_alloc_no_stats(unsigned int size, gfp_t gfp_extra_flag
>  	struct bpf_prog_aux *aux;
>  	struct bpf_prog *fp;
>  
> -	size = round_up(size, __PAGE_SIZE);
> +	size = round_up(size, PAGE_SIZE);
>  	fp = __vmalloc(size, gfp_flags);
>  	if (fp == NULL)
>  		return NULL;
> diff --git a/kernel/bpf/ringbuf.c b/kernel/bpf/ringbuf.c
> index e20b90c361316..8e4093ddbc638 100644
> --- a/kernel/bpf/ringbuf.c
> +++ b/kernel/bpf/ringbuf.c
> @@ -14,9 +14,9 @@
>  
>  #define RINGBUF_CREATE_FLAG_MASK (BPF_F_NUMA_NODE)
>  
> -/* non-mmap()'able part of bpf_ringbuf (everything up to consumer page) */
> +/* non-mmap()'able part of bpf_ringbuf (everything defined in struct) */
>  #define RINGBUF_PGOFF \
> -	(offsetof(struct bpf_ringbuf, consumer_pos) >> PAGE_SHIFT)
> +	(PAGE_ALIGN(sizeof(struct bpf_ringbuf)) >> PAGE_SHIFT)
>  /* consumer page and producer page */
>  #define RINGBUF_POS_PAGES 2
>  #define RINGBUF_NR_META_PAGES (RINGBUF_PGOFF + RINGBUF_POS_PAGES)
> @@ -69,10 +69,10 @@ struct bpf_ringbuf {
>  	 * validate each sample to ensure that they're correctly formatted, and
>  	 * fully contained within the ring buffer.
>  	 */
> -	unsigned long consumer_pos __aligned(PAGE_SIZE);
> -	unsigned long producer_pos __aligned(PAGE_SIZE);
> -	unsigned long pending_pos;
> -	char data[] __aligned(PAGE_SIZE);
> +	unsigned long *consumer_pos;
> +	unsigned long *producer_pos;
> +	unsigned long *pending_pos;
> +	char *data;
>  };
>  
>  struct bpf_ringbuf_map {
> @@ -134,9 +134,15 @@ static struct bpf_ringbuf *bpf_ringbuf_area_alloc(size_t data_sz, int numa_node)
>  	rb = vmap(pages, nr_meta_pages + 2 * nr_data_pages,
>  		  VM_MAP | VM_USERMAP, PAGE_KERNEL);
>  	if (rb) {
> +		void *base = rb;
> +
>  		kmemleak_not_leak(pages);
>  		rb->pages = pages;
>  		rb->nr_pages = nr_pages;
> +		rb->consumer_pos = (unsigned long *)(base + PAGE_SIZE * RINGBUF_PGOFF);
> +		rb->producer_pos = (unsigned long *)(base + PAGE_SIZE * (RINGBUF_PGOFF + 1));
> +		rb->pending_pos = rb->producer_pos + 1;
> +		rb->data = base + PAGE_SIZE * nr_meta_pages;
>  		return rb;
>  	}
>  
> @@ -179,9 +185,9 @@ static struct bpf_ringbuf *bpf_ringbuf_alloc(size_t data_sz, int numa_node)
>  	init_irq_work(&rb->work, bpf_ringbuf_notify);
>  
>  	rb->mask = data_sz - 1;
> -	rb->consumer_pos = 0;
> -	rb->producer_pos = 0;
> -	rb->pending_pos = 0;
> +	*rb->consumer_pos = 0;
> +	*rb->producer_pos = 0;
> +	*rb->pending_pos = 0;
>  
>  	return rb;
>  }
> @@ -300,8 +306,8 @@ static unsigned long ringbuf_avail_data_sz(struct bpf_ringbuf *rb)
>  {
>  	unsigned long cons_pos, prod_pos;
>  
> -	cons_pos = smp_load_acquire(&rb->consumer_pos);
> -	prod_pos = smp_load_acquire(&rb->producer_pos);
> +	cons_pos = smp_load_acquire(rb->consumer_pos);
> +	prod_pos = smp_load_acquire(rb->producer_pos);
>  	return prod_pos - cons_pos;
>  }
>  
> @@ -418,7 +424,7 @@ static void *__bpf_ringbuf_reserve(struct bpf_ringbuf *rb, u64 size)
>  	if (len > ringbuf_total_data_sz(rb))
>  		return NULL;
>  
> -	cons_pos = smp_load_acquire(&rb->consumer_pos);
> +	cons_pos = smp_load_acquire(rb->consumer_pos);
>  
>  	if (in_nmi()) {
>  		if (!spin_trylock_irqsave(&rb->spinlock, flags))
> @@ -427,8 +433,8 @@ static void *__bpf_ringbuf_reserve(struct bpf_ringbuf *rb, u64 size)
>  		spin_lock_irqsave(&rb->spinlock, flags);
>  	}
>  
> -	pend_pos = rb->pending_pos;
> -	prod_pos = rb->producer_pos;
> +	pend_pos = *rb->pending_pos;
> +	prod_pos = *rb->producer_pos;
>  	new_prod_pos = prod_pos + len;
>  
>  	while (pend_pos < prod_pos) {
> @@ -440,7 +446,7 @@ static void *__bpf_ringbuf_reserve(struct bpf_ringbuf *rb, u64 size)
>  		tmp_size = round_up(tmp_size + BPF_RINGBUF_HDR_SZ, 8);
>  		pend_pos += tmp_size;
>  	}
> -	rb->pending_pos = pend_pos;
> +	*rb->pending_pos = pend_pos;
>  
>  	/* check for out of ringbuf space:
>  	 * - by ensuring producer position doesn't advance more than
> @@ -460,7 +466,7 @@ static void *__bpf_ringbuf_reserve(struct bpf_ringbuf *rb, u64 size)
>  	hdr->pg_off = pg_off;
>  
>  	/* pairs with consumer's smp_load_acquire() */
> -	smp_store_release(&rb->producer_pos, new_prod_pos);
> +	smp_store_release(rb->producer_pos, new_prod_pos);
>  
>  	spin_unlock_irqrestore(&rb->spinlock, flags);
>  
> @@ -506,7 +512,7 @@ static void bpf_ringbuf_commit(void *sample, u64 flags, bool discard)
>  	 * new data availability
>  	 */
>  	rec_pos = (void *)hdr - (void *)rb->data;
> -	cons_pos = smp_load_acquire(&rb->consumer_pos) & rb->mask;
> +	cons_pos = smp_load_acquire(rb->consumer_pos) & rb->mask;
>  
>  	if (flags & BPF_RB_FORCE_WAKEUP)
>  		irq_work_queue(&rb->work);
> @@ -580,9 +586,9 @@ BPF_CALL_2(bpf_ringbuf_query, struct bpf_map *, map, u64, flags)
>  	case BPF_RB_RING_SIZE:
>  		return ringbuf_total_data_sz(rb);
>  	case BPF_RB_CONS_POS:
> -		return smp_load_acquire(&rb->consumer_pos);
> +		return smp_load_acquire(rb->consumer_pos);
>  	case BPF_RB_PROD_POS:
> -		return smp_load_acquire(&rb->producer_pos);
> +		return smp_load_acquire(rb->producer_pos);
>  	default:
>  		return 0;
>  	}
> @@ -680,12 +686,12 @@ static int __bpf_user_ringbuf_peek(struct bpf_ringbuf *rb, void **sample, u32 *s
>  	u64 cons_pos, prod_pos;
>  
>  	/* Synchronizes with smp_store_release() in user-space producer. */
> -	prod_pos = smp_load_acquire(&rb->producer_pos);
> +	prod_pos = smp_load_acquire(rb->producer_pos);
>  	if (prod_pos % 8)
>  		return -EINVAL;
>  
>  	/* Synchronizes with smp_store_release() in __bpf_user_ringbuf_sample_release() */
> -	cons_pos = smp_load_acquire(&rb->consumer_pos);
> +	cons_pos = smp_load_acquire(rb->consumer_pos);
>  	if (cons_pos >= prod_pos)
>  		return -ENODATA;
>  
> @@ -715,7 +721,7 @@ static int __bpf_user_ringbuf_peek(struct bpf_ringbuf *rb, void **sample, u32 *s
>  		 * Update the consumer pos, and return -EAGAIN so the caller
>  		 * knows to skip this sample and try to read the next one.
>  		 */
> -		smp_store_release(&rb->consumer_pos, cons_pos + total_len);
> +		smp_store_release(rb->consumer_pos, cons_pos + total_len);
>  		return -EAGAIN;
>  	}
>  
> @@ -737,9 +743,9 @@ static void __bpf_user_ringbuf_sample_release(struct bpf_ringbuf *rb, size_t siz
>  	 * prevents another task from writing to consumer_pos after it was read
>  	 * by this task with smp_load_acquire() in __bpf_user_ringbuf_peek().
>  	 */
> -	consumer_pos = rb->consumer_pos;
> +	consumer_pos = *rb->consumer_pos;
>  	 /* Synchronizes with smp_load_acquire() in user-space producer. */
> -	smp_store_release(&rb->consumer_pos, consumer_pos + rounded_size);
> +	smp_store_release(rb->consumer_pos, consumer_pos + rounded_size);
>  }
>  
>  BPF_CALL_4(bpf_user_ringbuf_drain, struct bpf_map *, map,