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Date:   Tue, 14 May 2019 11:42:11 +0800
From:   Jason Wang <>
To:     David Miller <>
Subject: Re: [PATCH net] vhost: don't use kmap() to log dirty pages

On 2019/5/14 上午12:42, David Miller wrote:
> From: Jason Wang <>
> Date: Mon, 13 May 2019 01:27:45 -0400
>> Vhost log dirty pages directly to a userspace bitmap through GUP and
>> kmap_atomic() since kernel doesn't have a set_bit_to_user()
>> helper. This will cause issues for the arch that has virtually tagged
>> caches. The way to fix is to keep using userspace virtual
>> address. Fortunately, futex has arch_futex_atomic_op_inuser() which
>> could be used for setting a bit to user.
>> Note there're several cases that futex helper can fail e.g a page
>> fault or the arch that doesn't have the support. For those cases, a
>> simplified get_user()/put_user() pair protected by a global mutex is
>> provided as a fallback. The fallback may lead false positive that
>> userspace may see more dirty pages.
>> Cc: Christoph Hellwig <>
>> Cc: James Bottomley <>
>> Cc: Andrea Arcangeli <>
>> Cc: Thomas Gleixner <>
>> Cc: Ingo Molnar <>
>> Cc: Peter Zijlstra <>
>> Cc: Darren Hart <>
>> Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
>> Signed-off-by: Jason Wang <>
> I want to see a review from Michael for this change before applying.

No problem, since kbuild spotted an issue. Let me post V2.


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