netdev - Re: [PATCH V5 0/9] Fixes for vhost metadata acceleration

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Date:   Tue, 13 Aug 2019 08:57:07 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jason Wang <jasowang@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH V5 0/9] Fixes for vhost metadata acceleration

On Tue, Aug 13, 2019 at 04:31:07PM +0800, Jason Wang wrote:

> What kind of issues do you see? Spinlock is to synchronize GUP with MMU
> notifier in this series.

A GUP that can't sleep can't pagefault which makes it a really weird
pattern

> Btw, back to the original question. May I know why synchronize_rcu() is not
> suitable? Consider:

We already went over this. You'd need to determine it doesn't somehow
deadlock the mm on reclaim paths. Maybe it is OK, the rcq_gq_wq is
marked WQ_MEM_RECLAIM at least..

I also think Michael was concerned about the latency spikes a long RCU
delay would cause.

Jason