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

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Date:   Mon, 13 May 2019 09:42:18 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     jasowang@...hat.com
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

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.