Re: [PATCH] mm/huge_memory: avoid transient none PMDs during lazyfree reclaim

[Zi Yan]

On Fri Oct 9, 2026 at 12:52 PM EDT, Kyle Zeng wrote:
> __discard_anon_folio_pmd_locked() clears and flushes a huge PMD before
> checking whether the folio can be discarded. If the folio was redirtied
> or has unexpected references, it restores the original PMD. Reclaim holds
> the PMD lock and the anon_vma read lock, but not the owning mmap_lock.
>
> Both zap_pmd_range() and mremap's get_old_pmd() can skip a none PMD
> without taking its lock. A concurrent munmap() or whole-VMA
> MREMAP_DONTUNMAP can therefore miss the PMD and later unlink the source
> VMA from its anon_vma, leaving a restored mapping behind. The anon_vma
> write lock taken by unlink_anon_vmas() waits for reclaim to finish, but
> does not repeat the skipped page-table walk. Once the remaining VMA
> links are removed, the folio's positive mapcount no longer guarantees a
> live anon_vma. Racing lazyfree reclaim against MREMAP_DONTUNMAP as an
> unprivileged user reproduces a KASAN use-after-free in
> folio_lock_anon_vma_read().
>
> Use pmdp_invalidate() to keep a recognizable, non-none huge PMD while
> the discard can still fail. Concurrent unmap and move operations then
> have to synchronize on the PMD lock instead of skipping the mapping.
> Clear the invalidated PMD only after the dirty and reference checks have
> succeeded, before removing the rmap and withdrawing the deposited page
> table. The full invalidation retains the TLB flush required for the
> dirty and GUP-fast checks.
>
> Fixes: 735ecdfaf4e8 ("mm/vmscan: avoid split lazyfree THP during shrink_folio_list()")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Codex:gpt-6-astra
> Signed-off-by: Kyle Zeng <kylebot@xxxxxxxxxx>
> ---
>  mm/huge_memory.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 1e5d68acf62a..20262195385a 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3569,11 +3569,17 @@ static bool __discard_anon_folio_pmd_locked(struct vm_area_struct *vma,
>  		return false;
>  	}
>  
> -	orig_pmd = pmdp_huge_clear_flush(vma, addr, pmdp);
> +	/*
> +	 * The discard may fail, so keep the PMD non-none until we're
> +	 * committed to discarding it. Otherwise, concurrent munmap() or
> +	 * mremap() can skip the PMD without taking the PTL and later unlink
> +	 * the VMA from its anon_vma despite a restored mapping.
> +	 */
> +	orig_pmd = pmdp_invalidate(vma, addr, pmdp);

There are some gaps we need to close before getting this fix in:

1. GUP-fast cannot follow the invalidated PMD: riscv and LoongArch need
pmd_access_permitted() that requires _PAGE_PRESENT; s390's
pmdp_invalidate() needs a change.

2. sparc64's thp_pte_count can be imbalanced with this change (based on
Lance's offlist feedback).

In addition, powerpc has a page table check issue similr to riscv, where riscv
fixed it with commit 9f4a88b8d01a6. This is not related to this issue
but discovered along with the investigation.

>  
>  	/*
>  	 * Syncing against concurrent GUP-fast:
> -	 * - clear PMD; barrier; read refcount
> +	 * - invalidate PMD; barrier; read refcount
>  	 * - inc refcount; barrier; read PMD
>  	 */
>  	smp_mb();
> @@ -3607,6 +3613,7 @@ static bool __discard_anon_folio_pmd_locked(struct vm_area_struct *vma,
>  		return false;
>  	}
>  
> +	pmdp_huge_get_and_clear(mm, addr, pmdp);
>  	folio_remove_rmap_pmd(folio, pmd_page(orig_pmd), vma);
>  	zap_deposited_table(mm, pmdp);
>  	add_mm_counter(mm, MM_ANONPAGES, -HPAGE_PMD_NR);




-- 
Best Regards,
Yan, Zi