Re: [PATCH v3 08/13] tracing: Add option to use memmapped memory for trace boot instance

[Steven Rostedt <rostedt@goodmis.org>]

Memory management folks. Please review this patch.

Specifically the "map_pages()" function below.

On Thu, 06 Jun 2024 17:17:43 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> 
> Add an option to the trace_instance kernel command line parameter that
> allows it to use the reserved memory from memmap boot parameter.
> 
>   memmap=12M$0x284500000 trace_instance=boot_mapped@0x284500000:12M
> 
> The above will reserves 12 megs at the physical address 0x284500000.
> The second parameter will create a "boot_mapped" instance and use the
> memory reserved as the memory for the ring buffer.
> 
> That will create an instance called "boot_mapped":
> 
>   /sys/kernel/tracing/instances/boot_mapped
> 
> Note, because the ring buffer is using a defined memory ranged, it will
> act just like a memory mapped ring buffer. It will not have a snapshot
> buffer, as it can't swap out the buffer. The snapshot files as well as any
> tracers that uses a snapshot will not be present in the boot_mapped
> instance.
> 
> Cc: linux-mm@kvack.org
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  .../admin-guide/kernel-parameters.txt         |  9 +++
>  kernel/trace/trace.c                          | 75 +++++++++++++++++--
>  2 files changed, 78 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index b600df82669d..ff26b6094e79 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6754,6 +6754,15 @@
>  			the same thing would happen if it was left off). The irq_handler_entry
>  			event, and all events under the "initcall" system.
>  
> +			If memory has been reserved (see memmap for x86), the instance
> +			can use that memory:
> +
> +				memmap=12M$0x284500000 trace_instance=boot_map@0x284500000:12M
> +
> +			The above will create a "boot_map" instance that uses the physical
> +			memory at 0x284500000 that is 12Megs. The per CPU buffers of that
> +			instance will be split up accordingly.
> +
>  	trace_options=[option-list]
>  			[FTRACE] Enable or disable tracer options at boot.
>  			The option-list is a comma delimited list of options
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 622fe670949d..13e89023f33b 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -9504,6 +9504,31 @@ static int instance_mkdir(const char *name)
>  	return ret;
>  }
>  
> +static u64 map_pages(u64 start, u64 size)
> +{
> +	struct page **pages;
> +	phys_addr_t page_start;
> +	unsigned int page_count;
> +	unsigned int i;
> +	void *vaddr;
> +
> +	page_count = DIV_ROUND_UP(size, PAGE_SIZE);
> +
> +	page_start = start;
> +	pages = kmalloc_array(page_count, sizeof(struct page *), GFP_KERNEL);
> +	if (!pages)
> +		return 0;
> +
> +	for (i = 0; i < page_count; i++) {
> +		phys_addr_t addr = page_start + i * PAGE_SIZE;
> +		pages[i] = pfn_to_page(addr >> PAGE_SHIFT);
> +	}
> +	vaddr = vmap(pages, page_count, VM_MAP, PAGE_KERNEL);
> +	kfree(pages);
> +
> +	return (u64)(unsigned long)vaddr;
> +}

If for some reason the memmap=nn$ss fails, but this still gets called,
will the above just map over any memory. That is, is it possible that
the kernel could have used this memory?

Is there a way to detect this? That is, I don't want this to succeed if
the memory location it's about to map to is used by the kernel, or will
be used by user space.

-- Steve


> +
>  /**
>   * trace_array_get_by_name - Create/Lookup a trace array, given its name.
>   * @name: The name of the trace array to be looked up/created.
> @@ -10350,6 +10375,7 @@ __init static void enable_instances(void)
>  {
>  	struct trace_array *tr;
>  	char *curr_str;
> +	char *name;
>  	char *str;
>  	char *tok;
>  
> @@ -10358,19 +10384,56 @@ __init static void enable_instances(void)
>  	str = boot_instance_info;
>  
>  	while ((curr_str = strsep(&str, "\t"))) {
> +		unsigned long start = 0;
> +		unsigned long size = 0;
> +		unsigned long addr = 0;
>  
>  		tok = strsep(&curr_str, ",");
> +		name = strsep(&tok, "@");
> +		if (tok) {
> +			start = memparse(tok, &tok);
> +			if (!start) {
> +				pr_warn("Tracing: Invalid boot instance address for %s\n",
> +					name);
> +				continue;
> +			}
> +		}
>  
> -		if (IS_ENABLED(CONFIG_TRACER_MAX_TRACE))
> -			do_allocate_snapshot(tok);
> +		if (start) {
> +			if (*tok != ':') {
> +				pr_warn("Tracing: No size specified for instance %s\n", name);
> +				continue;
> +			}
> +			tok++;
> +			size = memparse(tok, &tok);
> +			if (!size) {
> +				pr_warn("Tracing: Invalid boot instance size for %s\n",
> +					name);
> +				continue;
> +			}
> +			addr = map_pages(start, size);
> +			if (addr) {
> +				pr_info("Tracing: mapped boot instance %s at physical memory 0x%lx of size 0x%lx\n",
> +					name, start, size);
> +			} else {
> +				pr_warn("Tracing: Failed to map boot instance %s\n", name);
> +				continue;
> +			}
> +		} else {
> +			/* Only non mapped buffers have snapshot buffers */
> +			if (IS_ENABLED(CONFIG_TRACER_MAX_TRACE))
> +				do_allocate_snapshot(tok);
> +		}
>  
> -		tr = trace_array_get_by_name(tok, NULL);
> +		tr = trace_array_create_systems(name, NULL, addr, size);
>  		if (!tr) {
> -			pr_warn("Failed to create instance buffer %s\n", curr_str);
> +			pr_warn("Tracing: Failed to create instance buffer %s\n", curr_str);
>  			continue;
>  		}
> -		/* Allow user space to delete it */
> -		trace_array_put(tr);
> +
> +		/* Only allow non mapped buffers to be deleted */
> +		if (!start)
> +			trace_array_put(tr);
>  
>  		while ((tok = strsep(&curr_str, ","))) {
>  			early_enable_events(tr, tok, true);