linux-kernel - Re: [PATCH v2] vfio/pci: Handle concurrent vma faults

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Message-ID: <20210628185242.GI4459@nvidia.com>
Date:   Mon, 28 Jun 2021 15:52:42 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterx@...hat.com, prime.zeng@...ilicon.com, cohuck@...hat.com
Subject: Re: [PATCH v2] vfio/pci: Handle concurrent vma faults

On Mon, Jun 28, 2021 at 12:36:21PM -0600, Alex Williamson wrote:
> On Mon, 28 Jun 2021 14:30:28 -0300
> Jason Gunthorpe <jgg@...dia.com> wrote:
> 
> > On Mon, Jun 28, 2021 at 10:46:53AM -0600, Alex Williamson wrote:
> > > On Wed, 10 Mar 2021 11:58:07 -0700
> > > Alex Williamson <alex.williamson@...hat.com> wrote:
> > >   
> > > > vfio_pci_mmap_fault() incorrectly makes use of io_remap_pfn_range()
> > > > from within a vm_ops fault handler.  This function will trigger a
> > > > BUG_ON if it encounters a populated pte within the remapped range,
> > > > where any fault is meant to populate the entire vma.  Concurrent
> > > > inflight faults to the same vma will therefore hit this issue,
> > > > triggering traces such as:  
> > 
> > If it is just about concurrancy can the vma_lock enclose
> > io_remap_pfn_range() ?
> 
> We could extend vma_lock around io_remap_pfn_range(), but that alone
> would just block the concurrent faults to the same vma and once we
> released them they'd still hit the BUG_ON in io_remap_pfn_range()
> because the page is no longer pte_none().  We'd need to combine that
> with something like __vfio_pci_add_vma() returning -EEXIST to skip the
> io_remap_pfn_range(), but I've been advised that we shouldn't be
> calling io_remap_pfn_range() from within the fault handler anyway, we
> should be using something like vmf_insert_pfn() instead, which I
> understand can be called safely in the same situation.  That's rather
> the testing I was hoping someone who reproduced the issue previously
> could validate.

Yes, using the vmf_ stuff is 'righter' for sure, but there isn't
really a vmf for IO mappings..

> > I assume there is a reason why vm_lock can't be used here, so I
> > wouldn't object, though I don't especially like the loss of tracking
> > either.
> 
> There's no loss of tracking here, we were only expecting a single fault
> per vma to add the vma to our list.  This just skips adding duplicates
> in these cases where we can have multiple faults in-flight.  Thanks,

I mean the arch tracking of IO maps that is hidden inside ioremap_pfn

Jason