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

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Date:   Thu, 11 Mar 2021 11:35:53 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        prime.zeng@...ilicon.com, cohuck@...hat.com
Subject: Re: [PATCH] vfio/pci: Handle concurrent vma faults

On Thu, Mar 11, 2021 at 11:35:24AM +0000, Christoph Hellwig wrote:
> On Wed, Mar 10, 2021 at 03:06:07PM -0500, Peter Xu wrote:
> > On Wed, Mar 10, 2021 at 02:40:11PM -0400, Jason Gunthorpe wrote:
> > > On Wed, Mar 10, 2021 at 11:34:06AM -0700, Alex Williamson wrote:
> > > 
> > > > > I think after the address_space changes this should try to stick with
> > > > > a normal io_rmap_pfn_range() done outside the fault handler.
> > > > 
> > > > I assume you're suggesting calling io_remap_pfn_range() when device
> > > > memory is enabled,
> > > 
> > > Yes, I think I saw Peter thinking along these lines too
> > > 
> > > Then fault just always causes SIGBUS if it gets called
> 
> I feel much more comfortable having the io_remap_pfn_range in place.

It's just that Jason convinced me with the fact that io_remap_pfn_range() will
modify vma flags, and I tend to agree that's not a good thing to do during a
fault() handler (in remap_pfn_range):

	vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;

Although this case is special and it does not do harm it seems, since all these
four flags are already set by vfio_pci_mmap() anyways, so the flag didn't
really change at least with current code base.  It's just still cleaner to not
use io_remap_pfn_range() in vfio fault() since future change to the function
io_remap_pfn_range() may not guarantee to match with vfio mmap().

> 
> > 
> > Indeed that looks better than looping in the fault().
> > 
> > But I don't know whether it'll be easy to move io_remap_pfn_range() to device
> > memory enablement.  If it's a two-step thing, we can fix the BUG_ON and vma
> > duplication issue first, then the full rework can be done in the bigger series
> > as what be chosen as the last approach.
> 
> What kind of problems do you envision?  It seems pretty simple to do,
> at least when combined with the unmap_mapping_range patch.

Moving the prefault into device memory enablement will even remove the 1st
fault delay when doing the first MMIO access that triggers this fault().  Also
in that case I think we can also call io_remap_pfn_range() directly and safely,
rather than looping over vmf_insert_pfn_prot().

Thanks,

-- 
Peter Xu