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    SubjectRE: [patch 03/20] x86, ptrace, bts: defer branch trace stopping
    >-----Original Message-----
    >From: Peter Zijlstra [mailto:a.p.zijlstra@chello.nl]
    >Sent: Saturday, April 04, 2009 1:13 PM
    >To: Metzger, Markus T
    >Cc: mingo@elte.hu; tglx@linutronix.de; hpa@zytor.com; markus.t.metzger@gmail.com; roland@redhat.com;
    >eranian@googlemail.com; oleg@redhat.com; Villacis, Juan; ak@linux.jf.intel.com; linux-
    >kernel@vger.kernel.org; Andrew Morton
    >Subject: RE: [patch 03/20] x86, ptrace, bts: defer branch trace stopping
    >
    >On Sat, 2009-04-04 at 08:17 +0100, Metzger, Markus T wrote:
    >> >-----Original Message-----
    >> >From: Peter Zijlstra [mailto:a.p.zijlstra@chello.nl]
    >> >Sent: Friday, April 03, 2009 5:01 PM
    >> >To: Metzger, Markus T
    >>
    >>
    >> >Also, I can't say I like the name, what about something like:
    >> >
    >> >void account_locked_buffer(struct mm_struct *mm, long pages)
    >> >{
    >> > down_write(&mm->mmap_sem);
    >> >
    >> > mm->total_vm += pages;
    >> > mm->locked_vm += pages;
    >> >
    >> > up_write(&mm->mmap_sem);
    >> >}
    >> >
    >> >but looking more closely at that alloc_locked_buffer() stuff, I really
    >> >hate it, who in his right mind does a multi-page kmalloc() -- that's
    >> >crazy.
    >>
    >> I need a non-pageable chunk of memory to give to the cpu to store branch
    >> trace data in. Kmalloc() is easy to use and gives me what I need.
    >>
    >> How would I do this correctly?
    >
    >Well, how large should this buffer be and must it be physically
    >contiguous?

    The size is set by the user. It is limited by (locked memory) RLIMIT.

    The buffer need not be physically contiguous, but it must not be paged out
    and should be marked accessed and dirty.
    (see §18.7.8.2 in vol. 3b, Software Developer's Manual).

    >Apparently its large enough to account in pages, that would suggest you
    >should use the page allocator to get memory.

    Wouldn't kmalloc forward the request to the page allocator for large memory?


    >Furthermore, if it needs to be physically contiguous and its more than
    >say 8 pages worth, you're basically up shit creek.

    If not enough memory is available, I would expect users to request smaller
    buffers until the request can be satisfied.


    thanks and regards,
    markus.
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