Re: [PATCH 1/5] tracing, mm: Record pfn instead of pointer to struct page

[Vlastimil Babka <vbabka@suse.cz>]

On 04/14/2015 12:14 AM, Arnaldo Carvalho de Melo wrote:
> From: Namhyung Kim <namhyung@kernel.org>
> 
> The struct page is opaque for userspace tools, so it'd be better to save
> pfn in order to identify page frames.
> 
> The textual output of $debugfs/tracing/trace file remains unchanged and
> only raw (binary) data format is changed - but thanks to libtraceevent,
> userspace tools which deal with the raw data (like perf and trace-cmd)
> can parse the format easily.

Hmm it seems trace-cmd doesn't work that well, at least on current
x86_64 kernel where I noticed it:

 trace-cmd-22020 [003] 105219.542610: mm_page_alloc:        [FAILED TO PARSE] pfn=0x165cb4 order=0 gfp_flags=29491274 migratetype=1

I'm quite sure it's due to the "page=%p" part, which uses pfn_to_page().
The events/kmem/mm_page_alloc/format file contains this for page:

REC->pfn != -1UL ? (((struct page *)vmemmap_base) + (REC->pfn)) : ((void *)0)

I think userspace can't know vmmemap_base nor the implied sizeof(struct
page) for pointer arithmetic?

On older 4.4-based kernel:

REC->pfn != -1UL ? (((struct page *)(0xffffea0000000000UL)) + (REC->pfn)) : ((void *)0)

This also fails to parse, so it must be the struct page part?

I think the problem is, even if ve solve this with some more
preprocessor trickery to make the format file contain only constant
numbers, pfn_to_page() on e.g. sparse memory model without vmmemap is
more complicated than simple arithmetic, and can't be exported in the
format file.

I'm afraid that to support userspace parsing of the trace data, we will
have to store both struct page and pfn... or perhaps give up on reporting
the struct page pointer completely. Thoughts?

> So impact on the userspace will also be
> minimal.
> 
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> Based-on-patch-by: Joonsoo Kim <js1304@gmail.com>
> Acked-by: Ingo Molnar <mingo@kernel.org>
> Acked-by: Steven Rostedt <rostedt@goodmis.org>
> Cc: David Ahern <dsahern@gmail.com>
> Cc: Jiri Olsa <jolsa@redhat.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: linux-mm@kvack.org
> Link: http://lkml.kernel.org/r/1428298576-9785-3-git-send-email-namhyung@kernel.org
> Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
> ---
>  include/trace/events/filemap.h |  8 ++++----
>  include/trace/events/kmem.h    | 42 +++++++++++++++++++++---------------------
>  include/trace/events/vmscan.h  |  8 ++++----
>  3 files changed, 29 insertions(+), 29 deletions(-)
> 
> diff --git a/include/trace/events/filemap.h b/include/trace/events/filemap.h
> index 0421f49a20f7..42febb6bc1d5 100644
> --- a/include/trace/events/filemap.h
> +++ b/include/trace/events/filemap.h
> @@ -18,14 +18,14 @@ DECLARE_EVENT_CLASS(mm_filemap_op_page_cache,
>  	TP_ARGS(page),
>  
>  	TP_STRUCT__entry(
> -		__field(struct page *, page)
> +		__field(unsigned long, pfn)
>  		__field(unsigned long, i_ino)
>  		__field(unsigned long, index)
>  		__field(dev_t, s_dev)
>  	),
>  
>  	TP_fast_assign(
> -		__entry->page = page;
> +		__entry->pfn = page_to_pfn(page);
>  		__entry->i_ino = page->mapping->host->i_ino;
>  		__entry->index = page->index;
>  		if (page->mapping->host->i_sb)
> @@ -37,8 +37,8 @@ DECLARE_EVENT_CLASS(mm_filemap_op_page_cache,
>  	TP_printk("dev %d:%d ino %lx page=%p pfn=%lu ofs=%lu",
>  		MAJOR(__entry->s_dev), MINOR(__entry->s_dev),
>  		__entry->i_ino,
> -		__entry->page,
> -		page_to_pfn(__entry->page),
> +		pfn_to_page(__entry->pfn),
> +		__entry->pfn,
>  		__entry->index << PAGE_SHIFT)
>  );
>  
> diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
> index 4ad10baecd4d..81ea59812117 100644
> --- a/include/trace/events/kmem.h
> +++ b/include/trace/events/kmem.h
> @@ -154,18 +154,18 @@ TRACE_EVENT(mm_page_free,
>  	TP_ARGS(page, order),
>  
>  	TP_STRUCT__entry(
> -		__field(	struct page *,	page		)
> +		__field(	unsigned long,	pfn		)
>  		__field(	unsigned int,	order		)
>  	),
>  
>  	TP_fast_assign(
> -		__entry->page		= page;
> +		__entry->pfn		= page_to_pfn(page);
>  		__entry->order		= order;
>  	),
>  
>  	TP_printk("page=%p pfn=%lu order=%d",
> -			__entry->page,
> -			page_to_pfn(__entry->page),
> +			pfn_to_page(__entry->pfn),
> +			__entry->pfn,
>  			__entry->order)
>  );
>  
> @@ -176,18 +176,18 @@ TRACE_EVENT(mm_page_free_batched,
>  	TP_ARGS(page, cold),
>  
>  	TP_STRUCT__entry(
> -		__field(	struct page *,	page		)
> +		__field(	unsigned long,	pfn		)
>  		__field(	int,		cold		)
>  	),
>  
>  	TP_fast_assign(
> -		__entry->page		= page;
> +		__entry->pfn		= page_to_pfn(page);
>  		__entry->cold		= cold;
>  	),
>  
>  	TP_printk("page=%p pfn=%lu order=0 cold=%d",
> -			__entry->page,
> -			page_to_pfn(__entry->page),
> +			pfn_to_page(__entry->pfn),
> +			__entry->pfn,
>  			__entry->cold)
>  );
>  
> @@ -199,22 +199,22 @@ TRACE_EVENT(mm_page_alloc,
>  	TP_ARGS(page, order, gfp_flags, migratetype),
>  
>  	TP_STRUCT__entry(
> -		__field(	struct page *,	page		)
> +		__field(	unsigned long,	pfn		)
>  		__field(	unsigned int,	order		)
>  		__field(	gfp_t,		gfp_flags	)
>  		__field(	int,		migratetype	)
>  	),
>  
>  	TP_fast_assign(
> -		__entry->page		= page;
> +		__entry->pfn		= page ? page_to_pfn(page) : -1UL;
>  		__entry->order		= order;
>  		__entry->gfp_flags	= gfp_flags;
>  		__entry->migratetype	= migratetype;
>  	),
>  
>  	TP_printk("page=%p pfn=%lu order=%d migratetype=%d gfp_flags=%s",
> -		__entry->page,
> -		__entry->page ? page_to_pfn(__entry->page) : 0,
> +		__entry->pfn != -1UL ? pfn_to_page(__entry->pfn) : NULL,
> +		__entry->pfn != -1UL ? __entry->pfn : 0,
>  		__entry->order,
>  		__entry->migratetype,
>  		show_gfp_flags(__entry->gfp_flags))
> @@ -227,20 +227,20 @@ DECLARE_EVENT_CLASS(mm_page,
>  	TP_ARGS(page, order, migratetype),
>  
>  	TP_STRUCT__entry(
> -		__field(	struct page *,	page		)
> +		__field(	unsigned long,	pfn		)
>  		__field(	unsigned int,	order		)
>  		__field(	int,		migratetype	)
>  	),
>  
>  	TP_fast_assign(
> -		__entry->page		= page;
> +		__entry->pfn		= page ? page_to_pfn(page) : -1UL;
>  		__entry->order		= order;
>  		__entry->migratetype	= migratetype;
>  	),
>  
>  	TP_printk("page=%p pfn=%lu order=%u migratetype=%d percpu_refill=%d",
> -		__entry->page,
> -		__entry->page ? page_to_pfn(__entry->page) : 0,
> +		__entry->pfn != -1UL ? pfn_to_page(__entry->pfn) : NULL,
> +		__entry->pfn != -1UL ? __entry->pfn : 0,
>  		__entry->order,
>  		__entry->migratetype,
>  		__entry->order == 0)
> @@ -260,7 +260,7 @@ DEFINE_EVENT_PRINT(mm_page, mm_page_pcpu_drain,
>  	TP_ARGS(page, order, migratetype),
>  
>  	TP_printk("page=%p pfn=%lu order=%d migratetype=%d",
> -		__entry->page, page_to_pfn(__entry->page),
> +		pfn_to_page(__entry->pfn), __entry->pfn,
>  		__entry->order, __entry->migratetype)
>  );
>  
> @@ -275,7 +275,7 @@ TRACE_EVENT(mm_page_alloc_extfrag,
>  		alloc_migratetype, fallback_migratetype),
>  
>  	TP_STRUCT__entry(
> -		__field(	struct page *,	page			)
> +		__field(	unsigned long,	pfn			)
>  		__field(	int,		alloc_order		)
>  		__field(	int,		fallback_order		)
>  		__field(	int,		alloc_migratetype	)
> @@ -284,7 +284,7 @@ TRACE_EVENT(mm_page_alloc_extfrag,
>  	),
>  
>  	TP_fast_assign(
> -		__entry->page			= page;
> +		__entry->pfn			= page_to_pfn(page);
>  		__entry->alloc_order		= alloc_order;
>  		__entry->fallback_order		= fallback_order;
>  		__entry->alloc_migratetype	= alloc_migratetype;
> @@ -294,8 +294,8 @@ TRACE_EVENT(mm_page_alloc_extfrag,
>  	),
>  
>  	TP_printk("page=%p pfn=%lu alloc_order=%d fallback_order=%d pageblock_order=%d alloc_migratetype=%d fallback_migratetype=%d fragmenting=%d change_ownership=%d",
> -		__entry->page,
> -		page_to_pfn(__entry->page),
> +		pfn_to_page(__entry->pfn),
> +		__entry->pfn,
>  		__entry->alloc_order,
>  		__entry->fallback_order,
>  		pageblock_order,
> diff --git a/include/trace/events/vmscan.h b/include/trace/events/vmscan.h
> index 69590b6ffc09..f66476b96264 100644
> --- a/include/trace/events/vmscan.h
> +++ b/include/trace/events/vmscan.h
> @@ -336,18 +336,18 @@ TRACE_EVENT(mm_vmscan_writepage,
>  	TP_ARGS(page, reclaim_flags),
>  
>  	TP_STRUCT__entry(
> -		__field(struct page *, page)
> +		__field(unsigned long, pfn)
>  		__field(int, reclaim_flags)
>  	),
>  
>  	TP_fast_assign(
> -		__entry->page = page;
> +		__entry->pfn = page_to_pfn(page);
>  		__entry->reclaim_flags = reclaim_flags;
>  	),
>  
>  	TP_printk("page=%p pfn=%lu flags=%s",
> -		__entry->page,
> -		page_to_pfn(__entry->page),
> +		pfn_to_page(__entry->pfn),
> +		__entry->pfn,
>  		show_reclaim_flags(__entry->reclaim_flags))
>  );
>  
> 

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