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Date:   Tue, 26 May 2020 08:32:18 -0600
From:   Alex Williamson <alex.williamson@redhat.com>
To:     Peter Xu <peterx@redhat.com>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>, John Hubbard <jhubbard@nvidia.com>,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        cohuck@redhat.com, cai@lca.pw,
        Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH v3 3/3] vfio-pci: Invalidate mmaps and block MMIO access
 on disabled memory
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On Tue, 26 May 2020 09:49:54 -0400
Peter Xu <peterx@redhat.com> wrote:

> On Mon, May 25, 2020 at 09:37:05PM -0300, Jason Gunthorpe wrote:
> > On Mon, May 25, 2020 at 01:56:28PM -0700, John Hubbard wrote:  
> > > On 2020-05-25 09:56, Jason Gunthorpe wrote:  
> > > > On Mon, May 25, 2020 at 11:11:42AM -0400, Peter Xu wrote:  
> > > > > On Mon, May 25, 2020 at 11:46:51AM -0300, Jason Gunthorpe wrote:  
> > > > > > 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().  
> > > > > 
> > > > > Yeah.  My wild guess is that maybe fs code will always be without
> > > > > FAULT_FLAG_RETRY_NOWAIT so it's safe to sleep unconditionally (e.g., I think
> > > > > the general #PF should be fine to sleep in fault(); gup should be special, but
> > > > > I didn't observe any gup code called upon file systems)?  
> > > > 
> > > > get_user_pages is called on filesystem backed pages.
> > > > 
> > > > I have no idea what FAULT_FLAG_RETRY_NOWAIT is supposed to do. Maybe
> > > > John was able to guess when he reworked that stuff?
> > > >   
> > > 
> > > Although I didn't end up touching that particular area, I'm sure it's going
> > > to come up sometime soon, so I poked around just now, and found that
> > > FAULT_FLAG_RETRY_NOWAIT was added almost exactly 9 years ago. This flag was
> > > intended to make KVM and similar things behave better when doing GUP on
> > > file-backed pages that might, or might not be in memory.
> > > 
> > > The idea is described in the changelog, but not in the code comments or
> > > Documentation, sigh:
> > > 
> > > commit 318b275fbca1ab9ec0862de71420e0e92c3d1aa7
> > > Author: Gleb Natapov <gleb@redhat.com>
> > > Date:   Tue Mar 22 16:30:51 2011 -0700
> > > 
> > >     mm: allow GUP to fail instead of waiting on a page
> > > 
> > >     GUP user may want to try to acquire a reference to a page if it is already
> > >     in memory, but not if IO, to bring it in, is needed.  For example KVM may
> > >     tell vcpu to schedule another guest process if current one is trying to
> > >     access swapped out page.  Meanwhile, the page will be swapped in and the
> > >     guest process, that depends on it, will be able to run again.
> > > 
> > >     This patch adds FAULT_FLAG_RETRY_NOWAIT (suggested by Linus) and
> > >     FOLL_NOWAIT follow_page flags.  FAULT_FLAG_RETRY_NOWAIT, when used in
> > >     conjunction with VM_FAULT_ALLOW_RETRY, indicates to handle_mm_fault that
> > >     it shouldn't drop mmap_sem and wait on a page, but return VM_FAULT_RETRY
> > >     instead.  
> > 
> > So, from kvm's perspective it was to avoid excessively long blocking in
> > common paths when it could rejoin the completed IO by somehow waiting
> > on a page itself?
> > 
> > It all seems like it should not be used unless the page is going to go
> > to IO?  
> 
> I think NOWAIT is used as a common flag for kvm for its initial attempt to
> fault in a normal page, however...  I just noticed another fact that actually
> __get_user_pages() won't work with PFNMAP (check_vma_flags should fail), but
> KVM just started to support fault() for PFNMAP from commit add6a0cd1c5b (2016)
> using fixup_user_fault(), where nvidia seems to have a similar request to have
> a fault handler on some mapped BARs.
> 
> > 
> > Certainly there is no reason to optimize the fringe case of vfio
> > sleeping if there is and incorrect concurrnent attempt to disable the
> > a BAR.  
> 
> If fixup_user_fault() (which is always with ALLOW_RETRY && !RETRY_NOWAIT) is
> the only path for the new fault(), then current way seems ok.  Not sure whether
> this would worth a WARN_ON_ONCE(RETRY_NOWAIT) in the fault() to be clear of
> that fact.

Thanks for the discussion over the weekend folks.  Peter, I take it
you'd be satisfied if this patch were updated as:

diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
index aabba6439a5b..35bd7cd4e268 100644
--- a/drivers/vfio/pci/vfio_pci.c
+++ b/drivers/vfio/pci/vfio_pci.c
@@ -1528,6 +1528,13 @@ static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
 	struct vfio_pci_device *vdev = vma->vm_private_data;
 	vm_fault_t ret = VM_FAULT_NOPAGE;
 
+	/*
+	 * We don't expect to be called with NOWAIT and there are conflicting
+	 * opinions on whether NOWAIT suggests we shouldn't wait for locks or
+	 * just shouldn't wait for I/O.
+	 */
+	WARN_ON_ONCE(vmf->flags & FAULT_FLAG_RETRY_NOWAIT);
+
 	mutex_lock(&vdev->vma_lock);
 	down_read(&vdev->memory_lock);
 

Is that correct?  Thanks,

Alex