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

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Message-ID: <20190815193302.GT21596@ziepe.ca>
Date:   Thu, 15 Aug 2019 16:33:02 -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 Thu, Aug 15, 2019 at 11:26:46AM +0800, Jason Wang wrote:
> 
> On 2019/8/13 下午7:57, Jason Gunthorpe wrote:
> > 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
> 
> 
> My understanding is __get_user_pages_fast() assumes caller can fail or have
> fallback. And we have graceful fallback to copy_{to|from}_user().

My point is that if you can fall back to copy_user then it is weird to
call the special non-sleeping GUP under a spinlock.

AFAIK the only reason this is done is because of the way the notifier
is being locked...

Jason