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    SubjectRe: mapping user space buffer to kernel address space
    Hi,

    On Wed, Oct 18, 2000 at 03:23:17PM +0200, Andrea Arcangeli wrote:

    > This change makes sense and I agree it would cover the problem. However I
    > prefer to clarify that doing it for the swap cache as described is not nearly
    > enough to cover the mm corruption (everything that gets written via a memory
    > balancing mechanism should do the same).
    >
    > Said that I think it would be possible to do it for SHM and shared mappings too.

    shm already does it: shm_swap_core returns RETRY unless page count is
    1. file maps are close: file swapping already uses the address space
    a_ops, but currently uses that at the wrong time: unmap time rather
    than last-release. Rik?

    > However this still makes me wonder why should we unmap the pte of a page that
    > we can't free until we unmap_kiobuf? That's not as bad as having a
    > nopage+swapout dummy operations in the sound driver DMA page case, because
    > usually user-kiobufs are temporary just for the time of the DMA I/O, though.

    It's no different from mlock(): if you've got some reason why one
    specific mapping can't be unmapped (either it's locked or it is
    inherently non-swappable like a kiobuf), then there's no reason to
    make that per-mapping information visible at the per-page level.

    We allow multiple processes to mlock() the same page, after all.

    Cheers,
    Stephen
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