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Subject: Re: [PATCH v3 13/50] libperf: Lazily allocate mmap event copy
To:     Ian Rogers <irogers@google.com>
References: <20231024222353.3024098-1-irogers@google.com>
 <20231024222353.3024098-14-irogers@google.com>
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From:   Yang Jihong <yangjihong1@huawei.com>
Message-ID: <4d275fcd-a102-662f-d9f7-3acd3586acf2@huawei.com>
Date:   Wed, 25 Oct 2023 10:38:51 +0800
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Hello,

On 2023/10/25 6:23, Ian Rogers wrote:
> The event copy in the mmap is used to have storage to a read
> event. Not all users of mmaps read the events, such as perf record, so
> switch the allocation to being on first read rather than being
> embedded within the perf_mmap.
> 
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>   tools/lib/perf/include/internal/mmap.h | 2 +-
>   tools/lib/perf/mmap.c                  | 9 +++++++++
>   2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/lib/perf/include/internal/mmap.h b/tools/lib/perf/include/internal/mmap.h
> index 5a062af8e9d8..b11aaf5ed645 100644
> --- a/tools/lib/perf/include/internal/mmap.h
> +++ b/tools/lib/perf/include/internal/mmap.h
> @@ -33,7 +33,7 @@ struct perf_mmap {
>   	bool			 overwrite;
>   	u64			 flush;
>   	libperf_unmap_cb_t	 unmap_cb;
> -	char			 event_copy[PERF_SAMPLE_MAX_SIZE] __aligned(8);
> +	void			*event_copy;
>   	struct perf_mmap	*next;
>   };
>   
> diff --git a/tools/lib/perf/mmap.c b/tools/lib/perf/mmap.c
> index 2184814b37dd..91ae46aac378 100644
> --- a/tools/lib/perf/mmap.c
> +++ b/tools/lib/perf/mmap.c
> @@ -51,6 +51,8 @@ int perf_mmap__mmap(struct perf_mmap *map, struct perf_mmap_param *mp,
>   
>   void perf_mmap__munmap(struct perf_mmap *map)
>   {
> +	free(map->event_copy);
> +	map->event_copy = NULL;
>   	if (map && map->base != NULL) {
>   		munmap(map->base, perf_mmap__mmap_len(map));
>   		map->base = NULL;
> @@ -226,6 +228,13 @@ static union perf_event *perf_mmap__read(struct perf_mmap *map,
>   			unsigned int len = min(sizeof(*event), size), cpy;
>   			void *dst = map->event_copy;
>   
> +			if (!dst) {
> +				dst = malloc(PERF_SAMPLE_MAX_SIZE);
`event` is used as the return parameter of libperf API 
perf_mmap__read_event().
Is `zalloc` better to avoid dirty data?

Thanks,
Yang