Re: [PATCH] trace/events/page_ref: add page info to page_ref trace event

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 10/31/24 03:42, Kassey Li wrote:
> This followed
> commit 53d884a6675b ("mm, tracing: unify PFN format strings")
> to add page info.
> 
> In many kernel code we are talking with page other than pfn,
> here we added page algin with pfn.

How exactly would this help you, what are you doing with the trace?

> Signed-off-by: Kassey Li <quic_yingangl@quicinc.com>
> ---
>  include/trace/events/page_ref.h | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/include/trace/events/page_ref.h b/include/trace/events/page_ref.h
> index fe33a255b7d0..76df13b2a5b3 100644
> --- a/include/trace/events/page_ref.h
> +++ b/include/trace/events/page_ref.h
> @@ -18,6 +18,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_template,
>  
>  	TP_STRUCT__entry(
>  		__field(unsigned long, pfn)
> +		__field(const struct page *, page)
>  		__field(unsigned long, flags)
>  		__field(int, count)
>  		__field(int, mapcount)
> @@ -28,6 +29,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_template,
>  
>  	TP_fast_assign(
>  		__entry->pfn = page_to_pfn(page);

pfn is derived from the page, but not subject to KASLR, so in that sense is
better.
If you need to correlate the trace with some other data you obtained that
does contains page pointers, you will have to postprocess the trace with a
pfn_to_page() step, which is rather simple (but you'll need to obtain and
supply the randomized base) or have that other data source give you pfn too.

The tracepoints should not reveal the randomized base as trivially as they
would do after this patch.

> +		__entry->page = page;
>  		__entry->flags = page->flags;
>  		__entry->count = page_ref_count(page);
>  		__entry->mapcount = atomic_read(&page->_mapcount);
> @@ -36,8 +38,9 @@ DECLARE_EVENT_CLASS(page_ref_mod_template,
>  		__entry->val = v;
>  	),
>  
> -	TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d",
> +	TP_printk("pfn=0x%lx page=%p flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d",
>  		__entry->pfn,
> +		__entry->page,
>  		show_page_flags(__entry->flags & PAGEFLAGS_MASK),
>  		__entry->count,
>  		__entry->mapcount, __entry->mapping, __entry->mt,
> @@ -66,6 +69,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_and_test_template,
>  
>  	TP_STRUCT__entry(
>  		__field(unsigned long, pfn)
> +		__field(const struct page *, page)
>  		__field(unsigned long, flags)
>  		__field(int, count)
>  		__field(int, mapcount)
> @@ -77,6 +81,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_and_test_template,
>  
>  	TP_fast_assign(
>  		__entry->pfn = page_to_pfn(page);
> +		__entry->page = page;
>  		__entry->flags = page->flags;
>  		__entry->count = page_ref_count(page);
>  		__entry->mapcount = atomic_read(&page->_mapcount);
> @@ -86,8 +91,9 @@ DECLARE_EVENT_CLASS(page_ref_mod_and_test_template,
>  		__entry->ret = ret;
>  	),
>  
> -	TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d ret=%d",
> +	TP_printk("pfn=0x%lx page=%p flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d ret=%d",
>  		__entry->pfn,
> +		__entry->page,
>  		show_page_flags(__entry->flags & PAGEFLAGS_MASK),
>  		__entry->count,
>  		__entry->mapcount, __entry->mapping, __entry->mt,