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

[zhen.ni]

在 2026/5/9 08:29, SeongJae Park 写道:
> 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.

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