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 11:46:51 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Peter Xu <peterx@redhat.com>
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: <20200525144651.GE744@ziepe.ca>
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>
 <20200525142806.GC1058657@xz-x1>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200525142806.GC1058657@xz-x1>
User-Agent: Mutt/1.9.4 (2018-02-28)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, May 25, 2020 at 10:28:06AM -0400, Peter Xu wrote:
> 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

Sleeping looks fine, look at any FS implementation of fault, say,
xfs. The first thing it does is xfs_ilock() which does down_write().

I can't say when VM_FAULT_RETRY comes into play, but it is not so
simple as just sleeping..

Jason