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    Subjectpagefault_disable (was Re: [patch 6/6] mm: fix pagecache write deadlocks)
    (trimming cc list)

    Peter Zijlstra wrote:
    > On Sun, 2006-10-15 at 17:57 +0200, Nick Piggin wrote:

    >>Hmm, but you may not be doing a copy*user within the kmap. And you may
    >>want an atomic copy*user not within a kmap (maybe).
    >>
    >>I think it really would be more logical to do it in a wrapper function
    >>pagefault_disable() pagefault_enable()? ;)
    >
    >
    > I did that one first, but then noticed that most non trivial kmap_atomic
    > implementations already did the inc_preempt_count()/dec_preempt_count()
    > thing (except frv which did preempt_disable()/preempt_enable() ?)
    >
    > Anyway, here goes:
    >
    > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

    I think this is a good approach. The missed preempt checks could easily
    have been causing scheduling delays because the usercopy can take up a
    lot of kernel time.

    I don't know that the function should go in filemap.h... uaccess.h seems
    more appropriate, and had thought the pagefault_disable() be calle
    directly from within the copy_*_user_inatomic functions themselves, not
    the filemap helper.

    Also, the rest of the kernel tree (mainly uaccess and futexes) should be
    converted ;)

    > Index: linux-2.6/mm/filemap.h
    > ===================================================================
    > --- linux-2.6.orig/mm/filemap.h 2006-10-14 20:20:20.000000000 +0200
    > +++ linux-2.6/mm/filemap.h 2006-10-15 17:17:45.000000000 +0200
    > @@ -21,6 +21,22 @@ __filemap_copy_from_user_iovec_inatomic(
    > size_t bytes);
    >
    > /*
    > + * By increasing the preempt_count we make sure the arch preempt
    > + * handler bails out early, before taking any locks, so that the copy
    > + * operation gets terminated early.
    > + */
    > +pagefault_static inline void disable(void)
    > +{
    > + inc_preempt_count();
    > +}
    > +
    > +pagefault_static inline void enable(void)
    > +{
    > + dec_preempt_count();
    > + preempt_check_resched();
    > +}

    Interesting prototype ;)

    --
    SUSE Labs, Novell Inc.
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