linux-kernel - Mapping perf event files twice

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Message-ID: <ye8aauha1no.fsf@camel18.daimi.au.dk>
Date:	09 Mar 2010 17:40:59 +0100
From:	Soeren Sandmann <sandmann@...mi.au.dk>
To:	linux-kernel@...r.kernel.org
Subject: Mapping perf event files twice

Hi,

In the branch where perf_mmap is backed with vmalloc memory, this
code:

        data->user_page = all_buf;
        data->data_pages[0] = all_buf + PAGE_SIZE;
        data->data_order = ilog2(nr_pages);
        data->nr_pages = 1;

sets data->nr_pages to 1. This interferes with Sysprof which tries to
map the file twice consecutively to handle overflows automatically,
and therefore hits this in perf_mmap():

        if (atomic_inc_not_zero(&event->mmap_count)) {
                if (nr_pages != event->data->nr_pages)
                        ret = -EINVAL;
                goto unlock;
        }

Because event->data->nr_pages is 1 and Sysprof uses 32 pages, it gets
-EINVAL.

Possibly Sysprof shouldn't be playing this game, but if mapping the
file twice is not supported, then maybe perf_mmap() should just
explicitly forbid it.

Thanks,
Soren
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