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    SubjectRe: The disappearing sys_call_table export.
    Hi,

    I am interested with pt2, how NFS did for their syscall?

    Terje Eggestad wrote:
    > Unfortunately we live in an insane world.
    >
    > First of all, in the Changelog where the export was removed for 2.5.41
    >
    > http://www.kernel.org/pub/linux/kernel/v2.5/ChangeLog-2.5.41
    >
    > Arjan lists 4 reasons for having the export in the first place, and I'm
    > on point 3. Here Arjan pretty much acknowledges that there is a
    > legitimate need to have a event/hook system to be informed of a syscall.
    > The exact quote is: "Eg the use of the export in this just a bandaid due
    > to lack of a proper mechanism".
    >
    > My argument for *why* there should be a mechanism stops here.
    >
    >
    > Since you're bright inquisitive: The exact problem I'm facing is pretty
    > complex:
    >
    >
    > 1. performance is everything.
    > 2. We're making a MPI library, and as such we don't have any control
    > with the application.
    > 3a. The various hardware for cluster interconnect all work with DMA.
    > 3b. the performance loss from copying from a receive area to the
    > userspace buffer is unacceptable.
    > 3c. It's therefore necessary for HW to access user pages.
    > 4. In order to to 3, the user pages must be pinned down.
    > 5. the way MPI is written, it's not using a special malloc() to allocate
    > the send receive buffers. It can't since it would break language binding
    > to fortran. Thus ANY writeable user page may be used.
    > 6. point 4: pinning is VERY expensive (point 1), so I can't pin the
    > buffers every time they're used.
    > 7. The only way to cache buffers (to see if they're used before and
    > hence pinned) is the user space virtual address. A syscall, thus ioctl
    > to a device file is prohibitive expensive under point 1.
    > 8a. if the app (glibc in practice, but you never know) use sbrk() with a
    > negative arg, and then a positive argument, I can get a a different set
    > of user pages with the same address.
    > 8b ditto with a set of munmap()/mmap().
    > 9. since the number of times. any 'realloc' may happen is << than the
    > numbers of times any buffer may be used, it's necessary under point 1 to
    > to trace changes to virtual addresses to phys pages, rather than test
    > every time an address is being used.
    > 10. kernel patches are impractical, I must be able to do this with std
    > stock, redhat, AND suse kernels.
    >
    >
    >
    >
    > On Mon, 2003-05-05 at 10:23, Christoph Hellwig wrote:
    >
    >>On Mon, May 05, 2003 at 10:19:45AM +0200, Terje Eggestad wrote:
    >>
    >>>Now that it seem that all are in agreement that the sys_call_table
    >>>symbol shall not be exported to modules, are there any work in progress
    >>>to allow modules to get an event/notification whenever a specific
    >>>syscall is being called?
    >>
    >>No.
    >>
    >>
    >>>We have a specific need to trace mmap() and sbrk() calls.
    >>
    >>Well, you get mmap events for your driver and I can't imagine a sane
    >>reason for intwercepting sbrk(). Do you have a pointer to the driver
    >>source doing such strange things?


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