Re: [PATCH v5 1/3] mm/page_owner: add print_mode filter

[SeongJae Park]

On Thu,  7 May 2026 14:46:41 +0800 Zhen Ni <zhen.ni@xxxxxxxxxxxx> wrote:

> Add a print_mode filter to page_owner that allows users to choose between
> printing full stack traces or only stack handles, significantly reducing
> output size for debugging and analysis.
> 
> The filter provides a string-based interface under
> /sys/kernel/debug/page_owner_filter/:
> - Reading shows the current mode with [] brackets around active option
> - Writing accepts "full_stack" or "stack_handle" strings
> 
> The default full_stack mode maintains backward compatibility with existing
> usage, displaying complete stack traces for each page allocation.
> 
> The stack_handle mode dramatically reduces log size by showing only
> the handle number instead of the full stack trace. The mapping from
> handles to actual stack traces can be obtained via the
> show_stacks_handles interface.
> 
> Example usage:
>   # echo stack_handle > /sys/kernel/debug/page_owner_filter/print_mode
>   # cat /sys/kernel/debug/page_owner_filter/print_mode
>   full_stack [stack_handle]
>   # cat /sys/kernel/debug/page_owner
>   Page allocated via order 0, migratetype Unmovable, gfp_mask 0x1100ca,
>   pid 1, tgid 1 (systemd), ts 123456789 ns
>   PFN 0x1000 type Unmovable Block 1 type Unmovable
>   Flags 0x3fffe800000084(referenced|lru|active|private|node=0|zone=1)
>   handle: 17432583
>   ...
> 
> Signed-off-by: Zhen Ni <zhen.ni@xxxxxxxxxxxx>

I added a few trivial comments below, but overall looks good to me.

Reviewed-by: SeongJae Park <sj@xxxxxxxxxx>

> ---
> 
> Changes in v5:
> - No code changes
> 
> Changes in v4:
> - Change from numeric (0/1) to string-based interface ("full_stack"/"stack_handle")
> - Merge infrastructure patch into this patch
> 
> Changes in v3:
> - No code changes
> 
> Changes in v2:
> - Renamed from 'compact mode' to 'print_mode' for better clarity
> - Use enum values (0=full_stack, 1=stack_handle) instead of boolean
> - Update debugfs filename from 'compact' to 'print_mode'

Adding links to previous revisions would be helpful.

> ---
>  mm/page_owner.c | 93 +++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 91 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index 8178e0be557f..28766c854d02 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
[...]
> @@ -575,7 +594,11 @@ print_page_owner(char __user *buf, size_t count, unsigned long pfn,
>  			migratetype_names[pageblock_mt],
>  			&page->flags);
>  
> -	ret += stack_depot_snprint(handle, kbuf + ret, count - ret, 0);
> +	if (READ_ONCE(owner_filter.print_mode) == PAGE_OWNER_PRINT_STACK_HANDLE) {
> +		ret += scnprintf(kbuf + ret, count - ret,
> +				"handle: %d\n", handle);
> +	} else
> +		ret += stack_depot_snprint(handle, kbuf + ret, count - ret, 0);

Braces are unnecessary [2] because both branches have only one statement.

[...]
> +static ssize_t print_mode_write(struct file *file,
> +				 const char __user *buf,
> +				 size_t count, loff_t *ppos)
> +{
> +	char *kbuf;
> +	int mode;
> +	int ret = count;
> +
> +	/*
> +	 * Limit input size. Maximum valid input is "stack_handle" (12 chars)
> +	 * plus newline and null terminator. Use 32 bytes as a reasonable limit.
> +	 */
> +	if (count > 32)
> +		return -EINVAL;
> +
> +	kbuf = kmalloc(count + 1, GFP_KERNEL);
> +	if (!kbuf)
> +		return -ENOMEM;

Would it make sense to use kmalloc_objs(), or simply using a local array as
Andrew suggested?

[...]

[1] https://docs.kernel.org/process/submitting-patches.html#commentary
[2] https://docs.kernel.org/process/coding-style.html#placing-braces-and-spaces


Thanks,
SJ