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

[Lorenzo Stoakes (Oracle)]

On Wed, Mar 18, 2026 at 01:27:33PM +0000, Boone, Max wrote:
>
>
> > On Mar 18, 2026, at 2:08 PM, Max Boone <mboone@xxxxxxxxxx> wrote:
> >>
> >> 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?
>
> Forgot to mention, but yeah, we’re seeing this on Blackwell cards which have very
> large BARs, so probably seeing it first because of that. But the window was already
> pretty small, it’s not a very logical thing to poll numa_maps or smaps walks while the
> firmware of a VM is remapping the BARs of a GPU. With regards to that specific case
> there’s a proxmox thread and mail from the same person presumably [1, 2] that mentions
> the same bug.

No question we should take this fix, the page walk code is the right place to
check for this as we are not safe assuming the PUD entry can't change.

>
> [1] https://forum.proxmox.com/threads/walk_pgd_range-crash-pve9-1-on-6-18.179895/
> [2] https://lore.kernel.org/all/5948f3a6-8f30-4c45-9b86-2af9a6b37405@xxxxxxxxxx/

Cheers, Lorenzo