Re: [PATCH] mm/page_vma_mapped: continue past nonmatching huge PMDs

[Kyle Zeng]

On Thu, Oct 15, 2026 at 07:20:20PM -0700, Andrew Morton wrote:
> On Thu, 15 Oct 2026 18:15:49 -0700 Kyle Zeng <kylebot@xxxxxxxxxx> wrote:
> 
> > A PTE-mapped large folio can straddle PMD boundaries after an unaligned
> > mremap().  If the first part is discarded and replaced with an unrelated
> > huge PMD, page_vma_mapped_walk() stops instead of looking for the rest
> > of the folio in subsequent PMDs.
> > 
> > This also affects migration entries.  A walk that stops at an unrelated
> > huge PMD can leave migration PTEs behind when migration or an anonymous
> > folio split restores the mappings.  Those entries do not hold mapping
> > references and can outlive the source folio.
> > 
> > Skip the current PMD rather than ending the walk on a flag or PFN
> > mismatch.  Do the same if the PMD became nonpresent before the locked
> > recheck.  Release the PMD lock, clear the per-PMD state and advance with
> > step_forward(), so the existing range limit controls termination.
> > This covers present, migration and device-private huge PMDs while
> > preserving the matching-PMD and split-to-PTE paths.
> 
> As always, when fixing a bug please describe the worst-case
> userspace-visible effects of the bug.
> 
> I asked $LLM and was told
> 
> : The worst credible effects are permanent application hangs, kernel
> : crashes, and potentially system-wide denial of service.  Stale
> : migration entries can refer to freed and reused physical pages, causing
> : page faults to access unrelated folio metadata.  Memory corruption is
> : possible but unproven; cross-process information disclosure has not
> : been established.
> 
> But you have an LLM too!  So please never again send a bugfix without
> this info.
> 
> Also please explain whether there's a reproducer and if so, at least
> describe it and confirm the before-and-after results.
> 
> Thanks.
> 

Hi Andrew,

This issue can lead to memory corruption and I have a reproducer for it.
I can update the description and send in a v2 if you think it is proper.

Best,
Kyle