linux-kernel - Re: [PATCH net] vhost: don't use kmap() to log dirty pages

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Message-ID: <c8369a44-f4e0-4132-b470-cca7a044bb02@redhat.com>
Date:   Tue, 14 May 2019 11:42:11 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     David Miller <davem@...emloft.net>
Cc:     mst@...hat.com, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, hch@...radead.org,
        James.Bottomley@...senPartnership.com, aarcange@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, peterz@...radead.org,
        dvhart@...radead.org
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 <jasowang@...hat.com>
> 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 <hch@...radead.org>
>> Cc: James Bottomley <James.Bottomley@...senPartnership.com>
>> Cc: Andrea Arcangeli <aarcange@...hat.com>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Darren Hart <dvhart@...radead.org>
>> Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
> I want to see a review from Michael for this change before applying.


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

Thanks