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

From: Jason Wang
Date: Mon May 13 2019 - 23:43:58 EST

On 2019/5/14 äå12:42, David Miller wrote:
From: Jason Wang <jasowang@xxxxxxxxxx>
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@xxxxxxxxxxxxx>
Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Darren Hart <dvhart@xxxxxxxxxxxxx>
Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
I want to see a review from Michael for this change before applying.

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