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    SubjectRe: [PATCH] perf_counter: allow and require one-page mmap on counting counters
    Ingo Molnar writes:

    > * Paul Mackerras <paulus@samba.org> wrote:
    >
    > > +++ b/kernel/perf_counter.c
    > > @@ -1362,8 +1362,13 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
    > > vma_size = vma->vm_end - vma->vm_start;
    > > nr_pages = (vma_size / PAGE_SIZE) - 1;

    To answer your question below... this: ^^^

    > >
    > > - if (nr_pages == 0 || !is_power_of_2(nr_pages))
    > > - return -EINVAL;
    > > + if (counter->hw_event.record_type == PERF_RECORD_SIMPLE) {
    > > + if (nr_pages)
    > > + return -EINVAL;
    > > + } else {
    > > + if (nr_pages == 0 || !is_power_of_2(nr_pages))
    > > + return -EINVAL;
    > > + }
    >
    > Hm, is_power_of_2() is buggy then as 1 page is a power of two as
    > well: 1 == 2^0.
    >
    > Hm, it seems fine:
    >
    > static inline __attribute__((const))
    > bool is_power_of_2(unsigned long n)
    > {
    > return (n != 0 && ((n & (n - 1)) == 0));
    > }
    >
    > that should return true for an input of 1.
    >
    > What am i missing?
    >
    > Ingo

    We have one page as a header that contains the info for reading the
    counter value in userspace plus the head pointer, followed by (for a
    sampling counter) 2^N pages of ring buffer.

    Paul.


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