linux-kernel - Re: [PATCH v3 10/10] mm: kill vma flag VM_RESERVED and mm->reserved

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Date:	Tue, 09 Oct 2012 22:06:29 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
	Nick Piggin <npiggin@...nel.dk>
Subject: Re: [PATCH v3 10/10] mm: kill vma flag VM_RESERVED and
 mm->reserved_vm counter

On Tue, 2012-10-09 at 17:00 -0600, Alex Williamson wrote:
> On Tue, 2012-10-09 at 08:21 -0600, Alex Williamson wrote:
> > On Tue, 2012-10-09 at 21:12 +0900, Linus Torvalds wrote:
> > > On Tue, Oct 9, 2012 at 7:02 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > > >
> > > > It seems drivers/vfio/pci/vfio_pci.c uses VM_RESERVED
> > > 
> > > Yeah, I just pushed out what I think is the right (trivial) fix.
> > 
> > Thank you, looks correct to me as well.
> 
> Well, that might still be correct, but it's actually b3b9c293 (mm, x86,
> pat: rework linear pfn-mmap tracking) that breaks vfio.  As soon as I
> add that commit our use of mmap'd device areas stops working, both
> mapping them through the iommu and through kvm.  kvm hits the BUG_ON
> from !kvm_is_mmio_pfn in virt/kvm/kvm_main.c:hva_to_pfn.  Thanks,

It looks like vfio was for some reason relying on the cow special case
in remap_pfn_range() to set vma->vm_pgoff.  Since we're not doing a
is_cow_mapping, vm_pgoff is no longer updated in that function after
b3b9c293.  I'll add a vfio patch to fix this.  Thanks,

Alex

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