netdev - Re: [RFC PATCH V2] vhost: don't use kmap() to log dirty pages

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Message-ID: <e713f08b-b460-0094-fd28-f838c9efdff1@redhat.com>
Date:   Mon, 13 May 2019 13:22:37 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>
Subject: Re: [RFC PATCH V2] vhost: don't use kmap() to log dirty pages


On 2019/5/10 下午12:48, Jason Wang wrote:
>
> On 2019/5/10 上午10:59, Jason Wang wrote:
>>>>
>>>>         r = get_user_pages_fast(log, 1, 1, &page);
>>> OK so the trick is that page is pinned so you don't expect
>>> arch_futex_atomic_op_inuser below to fail. get_user_pages_fast
>>> guarantees page is not going away but does it guarantee PTE won't be
>>> invaidated or write protected?
>>
>>
>> Good point, then I think we probably need to do manual fixup through 
>> fixup_user_fault() if arch_futex_atomic_op_in_user() fail. 
>
>
> This looks like a overkill, we don't need to atomic environment here 
> actually. Instead, just keep pagefault enabled should work. So just 
> introduce arch_futex_atomic_op_inuser_inatomic() variant with 
> pagefault disabled there just for futex should be sufficient.
>
> Thanks


Ok, instead of using tricks, I think we can gracefully fallback to a 
get_user()/put_user() pair protected by a mutex.

Let me post a non-rfc version for this.

Thanks