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

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Message-ID: <20210312121611.07a313e3@omen.home.shazbot.org>
Date:   Fri, 12 Mar 2021 12:16:11 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterx@...hat.com, prime.zeng@...ilicon.com, cohuck@...hat.com
Subject: Re: [PATCH] vfio/pci: Handle concurrent vma faults

On Wed, 10 Mar 2021 14:40:11 -0400
Jason Gunthorpe <jgg@...dia.com> 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

Trying to use the address_space approach because otherwise we'd just be
adding back vma list tracking, it looks like we can't call
io_remap_pfn_range() while holding the address_space i_mmap_rwsem via
i_mmap_lock_write(), like done in unmap_mapping_range().  lockdep
identifies a circular lock order issue against fs_reclaim.  Minimally we
also need vma_interval_tree_iter_{first,next} exported in order to use
vma_interval_tree_foreach().  Suggestions?  Thanks,

Alex