Re: [PATCH] Fix perf mmap limitations on 32-bit.

[David Miller <davem@davemloft.net>]

From: David Miller <davem@davemloft.net>
Date: Sat, 10 Nov 2012 14:12:19 -0500 (EST)


Ping?

> This is a suggested patch to fix the bug I reported at:
> 
> 	http://marc.info/?l=linux-kernel&m=135033028924652&w=2
> 
> Essentially, there is a hard requirement that when perf analyzes a
> trace, it must have the entire thing mmap()'d.
> 
> Therefore the scheme used on 32-bit where we have a fixed (8) number
> of 32MB mmaps, and cycle through them, simply does not work.
> 
> One of the reasons this requirement exists is because the iterators
> maintain references to perf entry objects and those references don't
> just simply go away when this mmap code decides to cycle an old mmap
> area out and reuse it.  At this point, those entry pointers now point
> to garbage resulting in unpredictable behavior and crashes.
> 
> It is better to try to mmap() as much as we can and if we do actually
> run into address space limitations, the failure of the mmap() call
> will indicate that and stop processing.
> 
> I noticed that perf_session->mmap_window is set to a constant in one
> location, and only used in one other location.  So I got rid of it
> altogether.
> 
> So we adjust the size of the mmaps[] array to the maximum we could
> need.  On 64-bit we only need one slot.  On 32-bit we could need
> up to 128 (128 * 32MB == 4GB).
> 
> I've verified that this allows a large (~600MB) perf.data file to
> be analyzed properly with a 32-bit perf binary, which previously
> was not possible.
> 
> Signed-off-by: David S. Miller <davem@davemloft.net>
> 
> diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
> index 8cdd232..2cd3cc3 100644
> --- a/tools/perf/util/session.c
> +++ b/tools/perf/util/session.c
> @@ -128,15 +128,6 @@ struct perf_session *perf_session__new(const char *filename, int mode,
>  		goto out;
>  
>  	memcpy(self->filename, filename, len);
> -	/*
> -	 * On 64bit we can mmap the data file in one go. No need for tiny mmap
> -	 * slices. On 32bit we use 32MB.
> -	 */
> -#if BITS_PER_LONG == 64
> -	self->mmap_window = ULLONG_MAX;
> -#else
> -	self->mmap_window = 32 * 1024 * 1024ULL;
> -#endif
>  	self->machines = RB_ROOT;
>  	self->repipe = repipe;
>  	INIT_LIST_HEAD(&self->ordered_samples.samples);
> @@ -1369,6 +1360,18 @@ fetch_mmaped_event(struct perf_session *session,
>  	return event;
>  }
>  
> +/*
> + * On 64bit we can mmap the data file in one go. No need for tiny mmap
> + * slices. On 32bit we use 32MB.
> + */
> +#if BITS_PER_LONG == 64
> +#define MMAP_SIZE ULLONG_MAX
> +#define NUM_MMAPS 1
> +#else
> +#define MMAP_SIZE (32 * 1024 * 1024ULL)
> +#define NUM_MMAPS 128
> +#endif
> +
>  int __perf_session__process_events(struct perf_session *session,
>  				   u64 data_offset, u64 data_size,
>  				   u64 file_size, struct perf_tool *tool)
> @@ -1376,7 +1379,7 @@ int __perf_session__process_events(struct perf_session *session,
>  	u64 head, page_offset, file_offset, file_pos, progress_next;
>  	int err, mmap_prot, mmap_flags, map_idx = 0;
>  	size_t	page_size, mmap_size;
> -	char *buf, *mmaps[8];
> +	char *buf, *mmaps[NUM_MMAPS];
>  	union perf_event *event;
>  	uint32_t size;
>  
> @@ -1393,7 +1396,7 @@ int __perf_session__process_events(struct perf_session *session,
>  
>  	progress_next = file_size / 16;
>  
> -	mmap_size = session->mmap_window;
> +	mmap_size = MMAP_SIZE;
>  	if (mmap_size > file_size)
>  		mmap_size = file_size;
>  
> diff --git a/tools/perf/util/session.h b/tools/perf/util/session.h
> index dd64261..903966b 100644
> --- a/tools/perf/util/session.h
> +++ b/tools/perf/util/session.h
> @@ -29,7 +29,6 @@ struct ordered_samples {
>  struct perf_session {
>  	struct perf_header	header;
>  	unsigned long		size;
> -	unsigned long		mmap_window;
>  	struct machine		host_machine;
>  	struct rb_root		machines;
>  	struct perf_evlist	*evlist;