Re: [PATCH] tracing: Add boot-time backup of persistent ring buffer

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 21 Nov 2025 16:01:05 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
>  kernel/trace/trace.c |   58 +++++++++++++++++++++++++++++++++++++++++++++++---
>  kernel/trace/trace.h |    1 +
>  2 files changed, 55 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 33fcde91924c..616d24580371 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -10523,6 +10523,8 @@ static int __remove_instance(struct trace_array *tr)
>  		reserve_mem_release_by_name(tr->range_name);
>  		kfree(tr->range_name);
>  	}
> +	if (tr->flags & TRACE_ARRAY_FL_VMALLOC)
> +		vfree((void *)tr->range_addr_start);

The ring buffer user space mapping expects the ring buffer to be real
memory. Thus, virtual mapping is not supported. Either we need to allocate
normal memory, or the mmap function of his array must return an error.

>  
>  	for (i = 0; i < tr->nr_topts; i++) {
>  		kfree(tr->topts[i].topts);
> @@ -11328,6 +11330,41 @@ __init static void do_allocate_snapshot(const char *name)
>  static inline void do_allocate_snapshot(const char *name) { }
>  #endif
>  
> +__init static int backup_instance_area(const char *backup,
> +				       unsigned long *addr, phys_addr_t *size)
> +{
> +	struct trace_array *backup_tr;
> +	void *allocated_vaddr = NULL;
> +
> +	backup_tr = trace_array_get_by_name(backup, NULL);
> +	if (!backup_tr) {
> +		pr_warn("Tracing: Instance %s is not found.\n", backup);
> +		return -ENOENT;
> +	}

Add space.

> +	if (!(backup_tr->flags & TRACE_ARRAY_FL_BOOT)) {
> +		pr_warn("Tracing: Instance %s is not boot mapped.\n", backup);
> +		trace_array_put(backup_tr);
> +		return -EINVAL;
> +	}
> +
> +	*size = backup_tr->range_addr_size;
> +
> +	allocated_vaddr = vzalloc(*size);
> +	if (!allocated_vaddr) {
> +		pr_warn("Tracing: Failed to allocate memory for copying instance %s (size 0x%lx)\n",
> +			backup, (unsigned long)*size);
> +		trace_array_put(backup_tr);
> +		return -ENOMEM;
> +	}
> +
> +	memcpy(allocated_vaddr,
> +		(void *)backup_tr->range_addr_start, (size_t)*size);
> +	*addr = (unsigned long)allocated_vaddr;
> +
> +	trace_array_put(backup_tr);
> +	return 0;
> +}
> +
>  __init static void enable_instances(void)
>  {
>  	struct trace_array *tr;
> @@ -11350,11 +11387,15 @@ __init static void enable_instances(void)
>  		char *flag_delim;
>  		char *addr_delim;
>  		char *rname __free(kfree) = NULL;
> +		char *backup;
>  
>  		tok = strsep(&curr_str, ",");
>  
> -		flag_delim = strchr(tok, '^');
> -		addr_delim = strchr(tok, '@');
> +		name = strsep(&tok, "=");
> +		backup = tok;
> +
> +		flag_delim = strchr(name, '^');
> +		addr_delim = strchr(name, '@');
>  
>  		if (addr_delim)
>  			*addr_delim++ = '\0';
> @@ -11362,7 +11403,10 @@ __init static void enable_instances(void)
>  		if (flag_delim)
>  			*flag_delim++ = '\0';
>  
> -		name = tok;
> +		if (backup) {
> +			if (backup_instance_area(backup, &addr, &size) < 0)
> +				continue;
> +		}
>  
>  		if (flag_delim) {
>  			char *flag;
> @@ -11458,7 +11502,13 @@ __init static void enable_instances(void)
>  			tr->ref++;
>  		}
>  
> -		if (start) {
> +		/*
> +		 * Backup buffers can be freed but need vfree().
> +		 */
> +		if (backup)
> +			tr->flags |= TRACE_ARRAY_FL_VMALLOC;
> +
> +		if (start || backup) {
>  			tr->flags |= TRACE_ARRAY_FL_BOOT | TRACE_ARRAY_FL_LAST_BOOT;
>  			tr->range_name = no_free_ptr(rname);
>  		}

This would then need:

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index d1e527cf2aae..104e385e718e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -8791,8 +8791,8 @@ static int tracing_buffers_mmap(struct file *filp, struct vm_area_struct *vma)
 	struct trace_iterator *iter = &info->iter;
 	int ret = 0;
 
-	/* A memmap'ed buffer is not supported for user space mmap */
-	if (iter->tr->flags & TRACE_ARRAY_FL_MEMMAP)
+	/* A memmap'ed and backup buffers are not supported for user space mmap */
+	if (iter->tr->flags & (TRACE_ARRAY_FL_MEMMAP | TRACE_ARRAY_FL_VMALLOC))
 		return -ENODEV;
 
 	ret = get_snapshot_map(iter->tr);


As without it, when I tried mmapping the backup buffer I got this;

[  251.615441] BUG: unable to handle page fault for address: ffffd1c31e0000c8
[  251.615447] #PF: supervisor read access in kernel mode
[  251.615449] #PF: error_code(0x0000) - not-present page
[  251.615451] PGD 0 P4D 0 
[  251.615456] Oops: Oops: 0000 [#1] SMP PTI
[  251.615463] CPU: 6 UID: 0 PID: 1450 Comm: mmap-rb-lib Not tainted 6.18.0-rc3-ftest-00018-g02548113c2f6 #287 PREEMPT_{RT,LAZY} 
[  251.615469] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
[  251.615471] RIP: 0010:validate_page_before_insert+0x5/0x90
[  251.615479] Code: 48 c1 e8 0e 89 c2 83 e2 01 89 d0 c3 cc cc cc cc 0f 1f 44 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 0f 1f 44 00 00 <48> 8b 46 08 a8 01 75 5d 66 90 48 89 f0 8b 50 34 85 d2 74 68 48 89
[  251.615481] RSP: 0018:ffffa52b443d7988 EFLAGS: 00010246
[  251.615484] RAX: ffff8a84dcac4000 RBX: 00007f07e0c10000 RCX: 0000000108d2f067
[  251.615486] RDX: 0000000108d2f067 RSI: ffffd1c31e0000c0 RDI: ffff8a84dd865700
[  251.615487] RBP: 0000000000000000 R08: 000000005ab70715 R09: 0000000000000009
[  251.615489] R10: 0000000000000009 R11: ffff8a84c9320fb8 R12: ffff8a84dd865700
[  251.615490] R13: 0000000000000000 R14: ffffd1c31e0000c0 R15: ffff8a84c8d2f080
[  251.615492] FS:  00007f07e0e8e740(0000) GS:ffff8a86afbb7000(0000) knlGS:0000000000000000
[  251.615494] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  251.615496] CR2: ffffd1c31e0000c8 CR3: 000000011c390006 CR4: 0000000000170ef0
[  251.615501] Call Trace:
[  251.615502]  <TASK>
[  251.615504]  vm_insert_pages+0x157/0x360
[  251.615513]  __rb_map_vma+0x1ef/0x390
[  251.615521]  ring_buffer_map+0x261/0x370
[  251.615527]  tracing_buffers_mmap+0x7e/0x100
[  251.615532]  __mmap_region+0x831/0xe00
[  251.615554]  do_mmap+0x4bf/0x6b0
[  251.615560]  vm_mmap_pgoff+0x126/0x230
[  251.615569]  ksys_mmap_pgoff+0x162/0x220
[  251.615573]  do_syscall_64+0x76/0x9a0
[  251.615581]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[  251.615584] RIP: 0033:0x7f07e0f9dde2
[  251.615589] Code: 00 00 00 0f 1f 44 00 00 

-- Steve


> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 58be6d741d72..9e5186c96e9c 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -453,6 +453,7 @@ enum {
>  	TRACE_ARRAY_FL_LAST_BOOT	= BIT(2),
>  	TRACE_ARRAY_FL_MOD_INIT		= BIT(3),
>  	TRACE_ARRAY_FL_MEMMAP		= BIT(4),
> +	TRACE_ARRAY_FL_VMALLOC		= BIT(5),
>  };
>  
>  #ifdef CONFIG_MODULES