Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 25 May 2020 10:28:06 -0400
From:   Peter Xu <peterx@redhat.com>
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Alex Williamson <alex.williamson@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, cohuck@redhat.com, cai@lca.pw
Subject: Re: [PATCH v3 3/3] vfio-pci: Invalidate mmaps and block MMIO access
 on disabled memory
Message-ID: <20200525142806.GC1058657@xz-x1>
References: <159017449210.18853.15037950701494323009.stgit@gimli.home>
 <159017506369.18853.17306023099999811263.stgit@gimli.home>
 <20200523193417.GI766834@xz-x1>
 <20200523170602.5eb09a66@x1.home>
 <20200523235257.GC939059@xz-x1>
 <20200525122607.GC744@ziepe.ca>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20200525122607.GC744@ziepe.ca>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, May 25, 2020 at 09:26:07AM -0300, Jason Gunthorpe wrote:
> On Sat, May 23, 2020 at 07:52:57PM -0400, Peter Xu wrote:
> 
> > For what I understand now, IMHO we should still need all those handlings of
> > FAULT_FLAG_RETRY_NOWAIT like in the initial version.  E.g., IIUC KVM gup will
> > try with FOLL_NOWAIT when async is allowed, before the complete slow path.  I'm
> > not sure what would be the side effect of that if fault() blocked it.  E.g.,
> > the caller could be in an atomic context.
> 
> AFAICT FAULT_FLAG_RETRY_NOWAIT only impacts what happens when
> VM_FAULT_RETRY is returned, which this doesn't do?

Yes, that's why I think we should still properly return VM_FAULT_RETRY if
needed..  because IMHO it is still possible that the caller calls with
FAULT_FLAG_RETRY_NOWAIT.

My understanding is that FAULT_FLAG_RETRY_NOWAIT majorly means:

  - We cannot release the mmap_sem, and,
  - We cannot sleep

But we're allowed to return VM_FAULT_RETRY if any of the above is necessary.

Thanks,

-- 
Peter Xu