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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Fri, 17 Mar 2023 12:08:32 -0700
Message-ID: <CAJuCfpFkKuyBJkk8OzWEu2YCg-UYooS4bHuDaXvnCbeR-cBdVw@mail.gmail.com>
Subject: Re: [PATCH v4 0/7] introduce vm_flags modifier functions
To:     Alex Williamson <alex.williamson@redhat.com>
Cc:     akpm@linux-foundation.org, michel@lespinasse.org,
        jglisse@google.com, mhocko@suse.com, vbabka@suse.cz,
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On Tue, Mar 14, 2023 at 1:11=E2=80=AFPM Alex Williamson
<alex.williamson@redhat.com> wrote:
>
> On Thu, 26 Jan 2023 11:37:45 -0800
> Suren Baghdasaryan <surenb@google.com> wrote:
>
> > This patchset was originally published as a part of per-VMA locking [1]=
 and
> > was split after suggestion that it's viable on its own and to facilitat=
e
> > the review process. It is now a preprequisite for the next version of p=
er-VMA
> > lock patchset, which reuses vm_flags modifier functions to lock the VMA=
 when
> > vm_flags are being updated.
> >
> > VMA vm_flags modifications are usually done under exclusive mmap_lock
> > protection because this attrubute affects other decisions like VMA merg=
ing
> > or splitting and races should be prevented. Introduce vm_flags modifier
> > functions to enforce correct locking.
> >
> > The patchset applies cleanly over mm-unstable branch of mm tree.
>
> With this series, vfio-pci developed a bunch of warnings around not
> holding the mmap_lock write semaphore while calling
> io_remap_pfn_range() from our fault handler, vfio_pci_mmap_fault().
>
> I suspect vdpa has the same issue for their use of remap_pfn_range()
> from their fault handler, JasonW, MST, FYI.
>
> It also looks like gru_fault() would have the same issue, Dimitri.
>
> In all cases, we're preemptively setting vm_flags to what
> remap_pfn_range_notrack() uses, so I thought we were safe here as I
> specifically remember trying to avoid changing vm_flags from the
> fault handler.  But apparently that doesn't take into account
> track_pfn_remap() where VM_PAT comes into play.
>
> The reason for using remap_pfn_range() on fault in vfio-pci is that
> we're mapping device MMIO to userspace, where that MMIO can be disabled
> and we'd rather zap the mapping when that occurs so that we can sigbus
> the user rather than allow the user to trigger potentially fatal bus
> errors on the host.
>
> Peter Xu has suggested offline that a non-lazy approach to reinsert the
> mappings might be more inline with mm expectations relative to touching
> vm_flags during fault.  What's the right solution here?  Can the fault
> handling be salvaged, is proactive remapping the right approach, or is
> there something better?  Thanks,

Hi Alex,
If in your case it's safe to change vm_flags without holding exclusive
mmap_lock, maybe you can use __vm_flags_mod() the way I used it in
https://lore.kernel.org/all/20230126193752.297968-7-surenb@google.com,
while explaining why this should be safe?

>
> Alex
>