Re: [PATCH v3 13/50] libperf: Lazily allocate mmap event copy

[Ian Rogers]

On Tue, Oct 24, 2023 at 7:39 PM Yang Jihong <yangjihong1@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx>
> > ---
> >   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?

We could but I'd prefer not to zero unnecessarily. We've had similar
uninitialized memory being copied to perf.data file problems such as:
http://lore.kernel.org/lkml/20210309234945.419254-1-irogers@xxxxxxxxxx
With the sanitizers help we can avoid the problem and do less work,
which I think is best.

Thanks,
Ian

> Thanks,
> Yang