From: Alex Williamson <alex.williamson@redhat.com>
To: Suren Baghdasaryan <surenb@google.com>
Cc: akpm@linux-foundation.org, michel@lespinasse.org,
jglisse@google.com, mhocko@suse.com, vbabka@suse.cz,
hannes@cmpxchg.org, mgorman@techsingularity.net,
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linux-kernel@vger.kernel.org, kernel-team@android.com,
"Michael S. Tsirkin" <mst@redhat.com>,
"Jason Wang" <jasowang@redhat.com>,
"Jason Gunthorpe" <jgg@nvidia.com>,
"Cédric Le Goater" <clg@redhat.com>,
dimitri.sivanich@hpe.com
Subject: Re: [PATCH v4 0/7] introduce vm_flags modifier functions
Date: Fri, 17 Mar 2023 16:40:59 -0600 [thread overview]
Message-ID: <20230317164059.466d1c70.alex.williamson@redhat.com> (raw)
In-Reply-To: <CAJuCfpFkKuyBJkk8OzWEu2YCg-UYooS4bHuDaXvnCbeR-cBdVw@mail.gmail.com>
On Fri, 17 Mar 2023 12:08:32 -0700
Suren Baghdasaryan <surenb@google.com> wrote:
> On Tue, Mar 14, 2023 at 1:11 PM 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 facilitate
> > > the review process. It is now a preprequisite for the next version of per-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 merging
> > > 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?
Hi Suren,
Thanks for the reply, but I'm not sure I'm following. Are you
suggesting a bool arg added to io_remap_pfn_range(), or some new
variant of that function to conditionally use __vm_flags_mod() in place
of vm_flags_set() across the call chain? Thanks,
Alex
next prev parent reply other threads:[~2023-03-17 22:41 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-26 19:37 [PATCH v4 0/7] introduce vm_flags modifier functions Suren Baghdasaryan
2023-01-26 19:37 ` [PATCH v4 1/7] kernel/fork: convert vma assignment to a memcpy Suren Baghdasaryan
2023-01-26 19:40 ` Mike Rapoport
2023-01-26 19:37 ` [PATCH v4 2/7] mm: introduce vma->vm_flags wrapper functions Suren Baghdasaryan
2023-01-26 19:42 ` Mike Rapoport
2023-02-08 1:48 ` Hyeonggon Yoo
2023-01-26 19:37 ` [PATCH v4 3/7] mm: replace VM_LOCKED_CLEAR_MASK with VM_LOCKED_MASK Suren Baghdasaryan
2023-01-27 17:45 ` Davidlohr Bueso
2023-01-27 20:56 ` Suren Baghdasaryan
2023-01-26 19:37 ` [PATCH v4 4/7] mm: replace vma->vm_flags direct modifications with modifier calls Suren Baghdasaryan
2023-01-27 14:23 ` Liam R. Howlett
2023-01-31 8:32 ` Hyeonggon Yoo
2023-01-31 18:54 ` Suren Baghdasaryan
2023-01-31 20:53 ` Andrew Morton
2023-01-31 21:08 ` Suren Baghdasaryan
2023-01-31 23:12 ` Andrew Morton
2023-02-01 0:03 ` Suren Baghdasaryan
2023-02-01 12:23 ` Hyeonggon Yoo
2023-01-26 19:37 ` [PATCH v4 5/7] mm: replace vma->vm_flags indirect modification in ksm_madvise Suren Baghdasaryan
2023-01-26 22:45 ` Michael Ellerman
2023-02-08 1:54 ` Hyeonggon Yoo
2023-01-26 19:37 ` [PATCH v4 6/7] mm: introduce __vm_flags_mod and use it in untrack_pfn Suren Baghdasaryan
2023-01-26 19:44 ` Mike Rapoport
2023-01-26 19:37 ` [PATCH v4 7/7] mm: export dump_mm() Suren Baghdasaryan
2023-03-14 20:11 ` [PATCH v4 0/7] introduce vm_flags modifier functions Alex Williamson
2023-03-17 19:08 ` Suren Baghdasaryan
2023-03-17 22:40 ` Alex Williamson [this message]
2023-03-17 23:04 ` Suren Baghdasaryan
2023-03-22 13:48 ` Jason Gunthorpe
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