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

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Date:   Tue, 13 Aug 2019 09:41:05 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Jason Wang <jasowang@...hat.com>,
        "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 08:57:07AM -0300, 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

get_user_pages/get_user_pages_fast must not be called under a spinlock.
We have the somewhat misnamed __get_user_page_fast that just does a
lookup for existing pages and never faults for a few places that need
to do that lookup from contexts where we can't sleep.