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

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Message-ID: <20210628173028.GF4459@nvidia.com>
Date:   Mon, 28 Jun 2021 14:30:28 -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 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() ?

> IIRC, there were no blocking issues on this patch as an interim fix to
> resolve the concurrent fault issues with io_remap_pfn_range().
> Unfortunately it also got no Reviewed-by or Tested-by feedback.  I'd
> like to put this in for v5.14 (should have gone in earlier).  Any final
> comments?  Thanks,

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.

Jason