linux-kernel - Re: [PATCH v3 3/3] vfio-pci: Invalidate mmaps and block MMIO access on disabled memory

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Message-ID: <20200525151142.GE1058657@xz-x1>
Date:   Mon, 25 May 2020 11:11:42 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Alex Williamson <alex.williamson@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, cohuck@...hat.com, cai@....pw,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v3 3/3] vfio-pci: Invalidate mmaps and block MMIO access
 on disabled memory

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)?

Or I must have missed something important...

Thanks,

-- 
Peter Xu