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    Subject[PATCH v3-resend 10/11] kernel: drop voluntary schedule from might_fault
    might_fault is called from functions like copy_to_user
    which most callers expect to be very fast, like
    a couple of instructions. So functions like memcpy_toiovec call them
    many times in a loop.
    But might_fault calls might_sleep() and with CONFIG_PREEMPT_VOLUNTARY
    this results in a function call.

    Let's not do this - just call __might_sleep that produces
    a diagnostic for sleep within atomic, but drop
    might_preempt().

    Here's a test sending traffic between the VM and the host,
    host is built with CONFIG_PREEMPT_VOLUNTARY:
    Before:
    incoming: 7122.77 Mb/s
    outgoing: 8480.37 Mb/s
    after:
    incoming: 8619.24 Mb/s
    outgoing: 9455.42 Mb/s

    As a side effect, this fixes an issue pointed
    out by Ingo: might_fault might schedule differently
    depending on PROVE_LOCKING. Now there's no
    preemption point in both cases, so it's consistent.

    Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
    ---
    include/linux/kernel.h | 2 +-
    mm/memory.c | 3 ++-
    2 files changed, 3 insertions(+), 2 deletions(-)

    diff --git a/include/linux/kernel.h b/include/linux/kernel.h
    index e96329c..c514c06 100644
    --- a/include/linux/kernel.h
    +++ b/include/linux/kernel.h
    @@ -198,7 +198,7 @@ void might_fault(void);
    #else
    static inline void might_fault(void)
    {
    - might_sleep();
    + __might_sleep(__FILE__, __LINE__, 0);
    }
    #endif

    diff --git a/mm/memory.c b/mm/memory.c
    index 6dc1882..c1f190f 100644
    --- a/mm/memory.c
    +++ b/mm/memory.c
    @@ -4222,7 +4222,8 @@ void might_fault(void)
    if (segment_eq(get_fs(), KERNEL_DS))
    return;

    - might_sleep();
    + __might_sleep(__FILE__, __LINE__, 0);
    +
    /*
    * it would be nicer only to annotate paths which are not under
    * pagefault_disable, however that requires a larger audit and
    --
    MST

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