Re: [PATCH 01/46] mm/mglru: don't use stack pointers when walking PUD and PMD tables

[Baolin Wang]

On 10/27/26 7:17 AM, David Hildenbrand (Arm) wrote:
> From: Josef Bacik <josef@xxxxxxxxxxxxxx>
>
> MGLRU passes stack copies of page-table entries in walk_pud_range() and
> walk_pmd_range() when walking to the next level. These copies can become
> stale when khugepaged or MADV_DONTNEED (CONFIG_PT_RECLAIM) removes a PTE
> table concurrently.
>
> This pattern dates back to commit bd74fdaea146 ("mm: multi-gen LRU:
> support page table walks"). Likely, earlier locking masked the latent
> bug. Later changes exposed it: commit 1d65b771bc08 ("mm/khugepaged:
> retract_page_tables() without mmap or vma lock") allowed concurrent
> retraction, and commit cf4a65539c13 ("memcg-v1: no need for memcg locking
> for MGLRU") dropped the walker's outer RCU protection.
>
> The fix is simple, so just fix the original commit so we don't have to
> speculate. To fix it, pass the actual PMD/PUD pointers. This restores the
> RCU-protected lookup and validation against the live PMD. Note that the PUD
> handling is required as well to handle folded page tables correctly in all
> cases.
>
> Fix generated by an LLM and sent by Josef Bacik in private.
>
> Fixes: bd74fdaea146 ("mm: multi-gen LRU: support page table walks")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> [ david: handle walk_pud_range() as well, clarify+simplify description,
>           adjust Fixes: ]
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> ---

LGTM.
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>