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

["zhen.ni" <zhen.ni@easystack.cn>]

在 2026/5/9 08:29, SeongJae Park 写道:
> On Thu,  7 May 2026 14:46:41 +0800 Zhen Ni <zhen.ni@easystack.cn> 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@easystack.cn>
> 
> I added a few trivial comments below, but overall looks good to me.
> 
> Reviewed-by: SeongJae Park <sj@kernel.org>
> 
>> ---
>>
>> 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.

Good point. I will add lore links to all previous revisions in v6.

> 
>> ---
>>   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.
> 

Will fix in v6.

> [...]
>> +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?

I'll use a local array instead. Since the input is limited to 32 bytes,
a stack-allocated array should be sufficient and simpler:

     char kbuf[32 + 1];


Thanks for the review!
> 
> [...]
> 
> [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
> 
> 

Best regards,
Zhen