Re: [PATCH] mm/pagewalk: fix race between concurrent split and refault

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

On Wed, Mar 18, 2026 at 01:08:33PM +0000, Boone, Max wrote:
>
> > On Mar 18, 2026, at 1:55 PM, Lorenzo Stoakes (Oracle) <ljs@kernel.org> wrote:
> >
> >> […]
> >
> > So IOW, the PUD entry is split, then refaulted back to a PUD leaf entry
> > again?
>
> As far as I understand indeed, although the usage and faulting of huge
> pfnmaps does not feel intuitive to me yet. Empirically, yes, observing this
> when follow_fault_pfn() in drivers/vfio/vfio_iommu_type1.c is running
> concurrently with walk_pud_range(). I have another patch sent up to
> that list because this fix causes follow_fault_pfn() to return -EINVAL [1].

Ack

>
> >> […]
> >
> > I think it mirrors the retry logic in walk_pte_range() more closely right?
> > Because there it's:
> >
> > if (!pte)
> > walk->action = ACTION_AGAIN;
> > return err;
> >
> > I.e. let the parent handle the PTE not being got by pte_offset_map_lock(),
> > and you draw a comparison to this in the comment in walk_pmd_range().
>
> I’d personally say that the main logic introduced is walk_pud_range() retrying when
> walk_pmd_range() fails. We’re also splitting the PUD in walk_pud_range() and
> descending. But yeah, retry logic mirrors walk_pmd_range(), deciding that we need
> to retry mirrors walk_pte_range().

It's not a big deal we can leave that as is.

>
> >
> >>
> >> Fixes: a00cc7d9dd93 ("mm, x86: add support for PUD-sized transparent hugepages")
> >
> > Yikes, really? :) This is from 2017, I'm a little surprised we didn't hit
> > this bug until now.
> >
> > Has something changed more recently that made it more likely to hit? Or is
> > it one of those 'needed people to have more RAM first' or bigger PCI BAR's?
>
> Yeah, frankly, this is the first patch where I could find the splitting being introduced. It might
> be more correct to refer to the introduction of 1G huge_pfnmaps?

Yeah maybe that makes more sense? David - what do you think?

>
> >
> >> Cc: stable@vger.kernel.org
> >> Co-developed-by: David Hildenbrand (Arm) <david@kernel.org>
> >> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
> >> Signed-off-by: Max Boone <mboone@akamai.com>
> >
> > Only nits here, the logic LGTM, so:
>
> I’ll write up a PATCH v2 later today.

Cheers!

>
> >
> > […]
>
>

Thanks, Lorenzo