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    SubjectRe: locking question: do_mmap(), do_munmap()


    On Mon, 11 Oct 1999, Stephen C. Tweedie wrote:

    > Hi,
    >
    > On Mon, 11 Oct 1999 12:05:23 -0400 (EDT), Alexander Viro
    > <viro@math.psu.edu> said:
    >
    > > On Mon, 11 Oct 1999, Stephen C. Tweedie wrote:
    > >> No, spinlocks would be ideal. The vma swapout codes _have_ to be
    > >> prepared for the vma to be destroyed as soon as we sleep. In fact, the
    > >> entire mm may disappear if the process happens to exit. Once we know
    > >> which page to write where, the swapout operation becomes a per-page
    > >> operation, not per-vma.
    >
    > > Aha, so you propose to drop it in ->swapout(), right? (after get_file() in
    > > filemap_write_page()... Ouch. Probably we'ld better lambda-expand the call
    > > in filemap_swapout() - the thing is called from other places too)...
    >
    > Right now it is the big kernel lock which is used for this, and the
    > scheduler drops it anyway for us. If anyone wants to replace that lock
    > with another spinlock, then yes, the swapout method would have to drop
    > it before doing anything which could block. And that is ugly: having
    > spinlocks unbalanced over function calls is a maintenance nightmare.

    Agreed, but the big lock does not (and IMHO should not) cover the vma list
    modifications.


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