Re: user malloc from kernel

From: Terry Lambert (tlambert2_at_mindspring.com)
Date: 09/30/03

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    Date: Tue, 30 Sep 2003 01:13:35 -0700
To: earthman <earthman@inbox.ru>

    earthman wrote:
    > how to allocate some memory chunk
    > in user space memory from kernel code?
    > how to do it correctly?

    If your intent is to allocate a chunk of memory which is shared
    between your kernel and a single process in user space, the
    normal way of doing this is to allocate the memory to a device
    driver in the kernel, and then support mmap() on it to establish
    a user space mapping for the kernel allocated memory. In general,
    you must do this so that the memory is wired down in the kernel
    address space, so that if you attempt to access it in the kernel
    while the process you are interested in sharing with is swapped
    out, you do not segfault and trap-12 ("page not present") panic
    your kernel.

    If your intent is to share memory with every process in user
    space (e.g. similar to what some OS's use to implement zero
    system call gettimeofday() functions, etc.), then you want to
    allocate the memory in kernel space (still), make sure it's on
    a page boundary, and set the PG_G and PG_U bits on the page(s)
    in question.

    -- Terry
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